Best colour for a dog to track an object against green background

Best colour for a dog to track an object against green background

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As dogs have limited colour perception, what colour would appear with most contrast against a green background for a dog? I understand that red and green are very close in a dog's colour perception, so would blue offer better contrast?

Dogs are a dichromatic species, featuring only a long wavelength (L) and a short wavelength (S) cone (source: Smithsonian). As such, they are thought to perceive mainly blues and yellowish hues (Fig. 1). This is unlike trichromatic species like humans, who are able to distinguish red and greens as well (Fig. 2). Whether dogs perceive greens or yellows is hard to say, but either way, they see those longer wavelengths (red, green, yellow) as one hue (say, yellowish).

Hence, in dogs, green hues are basically seen as yellow (or vice versa) and indeed blue (short wavelengths) are best to create a good color contrast.

Of course, brightness contrast will work fine too (white and black).

Fig.1. Dichromatic (dog) color spectrum. source: Dog Vision

Fig. 2. Trichromatic (human) color spectrum. source: Dog Vision

You can use the mask to index the array, and assign just the white parts of the mask to white:

I really recommend you to stick with OpenCV, it is well optimized. The trick is to invert the mask and apply it to some background, you will have your masked image and a masked background, then you combine both. image1 is your image masked with the original mask, image2 is the background image masked with the inverted mask, and image3 is the combined image. Important. image1, image2 and image3 must be of the same size and type. The mask must be grayscale.

At first, you need to get the background. To this must be subtracted from the original image with the mask image. And then change the black background to white (or any color). And then back to add with the image of the mask. Look here OpenCV grabcut() background color and Contour in Python

First convert to GRAY and then threshold with cv2.threshold and then use numpy masking.

Unless you have a very wide focal length, you can generally track in a background using just a single marker if the camera only pan/tilts.

ISSUES: Make sure the marker doesn't get covered, or you're in trouble! It's better to always have more than one for safety. The green screen cloth is also too wrinkly &mdash cloth only works if it's properly suspended on a frame. The marker isn't blue for any particular reason &mdash we just had some Rosco tape lying around.

TRY THIS: Notice the shadow going over her. If you put her on a sunny background, it seems totally motivated, and helps the realism.

TECH SPECS: 1080p, 30fps, Unknown green screen cloth, Sharpness -7, Cine-Like Matrix.

Can Dogs See in Ultraviolet?

Research suggests that your dog may be able to see things that are completely invisible to you.

If you look at the size, shape, and general structure of a dog's eye it looks very much like the human eye. For that reason we have a tendency to guess that vision in dogs is much like that in humans. However science has been advancing and we are learning that dogs and humans don't always see the same thing and don't always have the same visual abilities. For example, although dogs do have some color vision (click here for more about that) their range of colors is much more limited in comparison to humans. Dogs tend to see the world in shades of yellow, blue, and gray and can't discriminate between the colors that we see as red and green. Humans also have better visual acuity, and can discriminate details that dogs cannot (click here to read more about that).

On the flip side, the dog's eye is specialized for night vision and canines can see more under dim lighting than we humans can. Furthermore, dogs can see motion better than people. However a study published in the Proceedings of the Royal Society B* suggests that dogs may also see a whole range of visual information that humans cannot.

Ronald Douglas, a professor of biology at City University London and Glenn Jeffrey, a professor of neuroscience at University College London, were interested in seeing whether mammals could see in the ultraviolet light range. The wave lengths of visible light are measured in nanometers (a nanometer is one millionth of one thousandth of a meter). The longer wave lengths, around 700 nm, are seen by humans as red, and the shorter wavelengths, around 400 nm, are seen as blue or violet. Wavelengths of light which are shorter than 400 nm are not seen by normal humans, and light in this range is called ultraviolet.

It is well known that some animals, such as insects, fish, and birds, can see in the ultraviolet. For bees this is a vital ability. When humans look at certain flowers they might see something which has a uniform color, however many species of flowers have adapted their coloration so that when viewed with ultraviolet sensitivity the center of the flower (which contains the pollen and nectar) is a readily visible target making it easier for a bee to find. You can see that in this figure.

In human beings the lens inside the eye has a yellowish tint which filters out the ultraviolet light. The British research team reasoned that certain other species of mammals might not have such yellowish components in their eyes and therefore might be sensitive to ultraviolet light. It is certainly the case that people who have had the lens of their eye removed surgically because of cataracts often report a change in their vision. With the removal of the yellowish lens such individuals can now see in the ultraviolet range. For example, some experts believe that it was because of such a cataract operation the artist Monet began to paint flowers with a blue tinge.

In the current study a broad range of animals including: dogs, cats, rats, reindeer, ferrets, pigs, hedgehogs and many others, were tested. The transparency of the optical components of their eyes was measured and it was found that a number of these species did allow a good deal of ultraviolet light into their eyes. When the eye of the dog was tested they found that it allowed over 61% of the UV light to pass through and reach the photosensitive receptors in the retina. Compare this to humans where virtually no UV light gets through. With this new data we can determine how a dog might see a visual spectrum (like a rainbow) in comparison to a human and that is simulated in this figure.

The obvious question to ask is what benefits the dog derives from its ability to see in the ultraviolet. It may have something to do with having an eye that is adapted so that it has good night vision, since it appears that those species who were at least partially nocturnal had lenses capable of transmitting ultraviolet, while those who functioned mostly in the daylight did not. However it is also the case that certain types of information can be processed if you have ultraviolet sensitivity. Anything that either absorbs the ultraviolet or reflects it differentially would thus become visible. For example in this figure we have an individual on whom we have painted a pattern using a sunscreen lotion (which blocks ultraviolet). The pattern is not visible under normal conditions, but when viewed in ultraviolet light it becomes quite clear.

In nature there are a number of significant things which might become visible if you can see in the ultraviolet. Of interest to dogs is the fact that urine trails become visible in ultraviolet. Since urine is used by dogs to learn something about other dogs in their environment, it may be useful to be able to spot patches of it easily. This might also be of assistance in wild canines as a method of spotting and trailing potential prey.

In certain specific environments sensitivity to the ultraviolet part of the spectrum can provide an advantage to an animal that hunts in order to survive, such as the ancestors of our dogs. Consider the figure below. You can see that the white coloration of an arctic hare provides good camouflage and makes the animal difficult to spot against a snowy background. However such camouflage is not as good when used against an animal with ultraviolet visual capacities. This is because the snow will reflect much of the ultraviolet light while white fur does not reflect the UV rays as well. Thus to the UV sensitive eye the arctic hare is now much more easily seen since it appears as if it is a lightly shadowed form, rather than white against white, as can be seen in the simulation below.

If visual sensitivity in the ultraviolet does provide certain advantages to an animal like a dog, then perhaps the question we should be asking is why other animals, like humans, would not benefit as well from having the ability to register ultraviolet light. The answer seems to come from the fact that there are always trade-offs in vision. You can have an eye that is sensitive in low levels of light, such as the dog's eye, but that sensitivity comes at a cost. It is the short wavelengths of light (those that we see as blue, and even more so, those shorter yet wavelengths that we call ultraviolet) which are most easily scattered as they enter the eye. This light scattering degrades the image and makes it blurry so you can't see details. So dogs who evolved from nocturnal hunters may have maintained their ability to see ultraviolet light because they need that sensitivity when there is little light around. Animals who function in the daylight, such as we humans, rely more on our visual acuity to effectively deal with the world. So we have eyes that screen out the ultraviolet in order to improve our ability to see fine visual details.

We have been talking about the first study which has dealt with this aspect of canine vision and its results were a surprise to many of us who never expected that dogs might have this added form of visual sensitivity. Obviously further research is needed to determine how dogs really benefit from this ability. I doubt that it was an evolutionary development which simply allows dogs to have greater appreciation for the psychedelic posters which became so popular in the 1970s — you know those posters that were created by using inks that fluoresced under a "black light" or ultraviolet light source. But only through future research will we know for sure.

Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission

* Data from: R. H. Douglas, G. Jeffery (2014). The writer spectral transmission of ocular media suggest ultraviolet sensitivity is widespread among mammals. Proceedings of the Royal Society B, April, volume 281, issue 1780.

What is a green screen anyways?

Also called "chroma-keying," filming footage against a green screen allows you to layer two separate shots together. For example, if you can’t shoot on location, you can film your subject against a green screen and drop in a background after the shoot. Green and blue colors were originally chosen for the background because human skin tones don’t have any blue or green, making it easy for actors to stand out. Today, chroma keys can technically be any color, but green is still the most common, which is why it’s called a “green” screen. Many of our customers use green screen footage to create a more polished look and feel in Youtube videos - for example, by compositing a professional studio background behind an interview subject. Whatever your project, green screens are an essential tool in any video editor’s toolkit

Electronic supplementary material is available online at

Published by the Royal Society under the terms of the Creative Commons Attribution License, which permits unrestricted use, provided the original author and source are credited.


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Color is a function of the human visual system, and is not an intrinsic property. Objects don't have a color, they give off light that appears to be a color. Spectral power distributions exist in the physical world, but color exists only in the mind of the beholder. Our perception of color is not an objective measure of anything about the light that enters our eyes, but it correlates pretty well with objective reality.

Color is determined first by frequency and then by how those frequencies are combined or mixed when they reach they eye. This is the physics part of the topic. Light falls on specialized receptor cells (called cones) at the back of the eye (called the retina) and a signal is sent to the brain along a neural pathway (called the optic nerve). This signal is processed by the part of the brain near the back of the skull (called the occipital lobe). Here's where the biology kicks in, or maybe it's the psychology, or maybe it's both. They eye is very much like a camera, but the brain is not at all like a video recorder. The brain is not like a computer with fixed hardware of transistors and capacitors executing some sort of software code. The neurons of the brain are probably best thought of as wetware — a fusion of hardware and software or maybe something completely different. I don't feel qualified to say much about that end of this process. Once the visual information leaves the eye, basic physics ends and neurocognition takes over.

Color is determined first by frequency. Let's start by determining what a typical person would see when looking at electromagnetic radiation of a single frequency. Physicists call this light. (The literal meaning of this word is "single color", but the actual meaning is "single frequency".)

Low frequency radiation is invisible. With an adequately bright source, starting somewhere around 400 THz (1 THz = 10 12 Hz) most humans begin to perceive a dull red. As the frequency is increased, the perceived color gradually changes from red to orange to yellow to green to blue to violet. The eye doesn't perceive violet so well. It always seems to look dark compared to other sources at equal intensity. Somewhere between 700 THz and 800 THz the world goes dark again.

How many colors are there in the spectrum above? How many did I name?

red orange yellow green blue violet

The simple named colors are mostly monosyllabic English words — red , green, brown, black, white, gray. Brevity indicates an Old English (Anglo-Saxon) origin. Monosyllabic words are generally the oldest words in the English language — head, eye, nose, foot, cat, dog, cow, eat, drink, man, wife, house, sleep, rain, snow, sword, sheath, God…. These words go back more than fifteen centuries. Yellow, purple, and blue are exceptions to the one-syllable-equals-English rule. Yellow and purple are Old English color words with two syllables. Blue is a one syllable French word (bleu) that replaced a two syllable Old English word (hǽwen) eight hundred years ago.

Some of the names for colors are loan words from French (many of which are loan words from other languages). Since the ʒ (zh) sound doesn't exist in Old English, orange and beige are obviously French. (Garage is also an obviously French word.) The words violet and orange were the names of plants (nouns) before they were the names of colors (adjectives). Violet came from 14th century French, which came from Latin. Orange came from 16th century French, which came from Italian, which came from Arabic, which came from Persian, which came from Sanskrit.

English arose when three Germanic tribes — Angles, Saxons, and Jutes — migrated from continental Europe to the British Isles in the 5th century. The language they spoke is called Anglo-Saxon or Old English. You would hardly recognize this language if you heard it spoken or saw it written today. Danes probably have the best chance of understanding spoken Old English, Icelanders the best chance of understanding written Old English. Of the six named colors in my spectrum, only four were known to the Anglo-Saxons: reád, geolu, grÉne, hǽwen. Do you recognize any of them?

In the year 1066, an invasion of French speaking peoples — Normans, Bretons, and French — swept over the British Isles. The last Anglo-Saxon King of England, King Harold II, was succeeded by the first Norman king, William the Conqueror. The Normans had an odd empire (if that's even the word for it) that included the British Isles, northern France (appropriately named Normandy), southern Italy, Sicily, Syria, Cyprus, and Libya. William was a Norman, descended from Norsemen, but he spoke French not Swedish or Norwegian or Danish. One factor leading to the rise of the Normans in their scattered empire is their ability to quickly integrate themselves into the culture of the peoples they conquered. For purposes of this discussion, we care about language. When the Normans got to northern France, they started speaking French. When the Normans got to England they got the Anglo-Saxons to start speaking French too (sort of). In about a hundred years, Anglo-Saxon had mutated into something closer to what we would recognize as English today — neither French nor Anglo-Saxon. Old English became Middle English. This is when English acquired the words blue (which replaced hǽwen) and violet (which never existed as an English color word before).

rede ȝeoluw grene blu violet

The next change in the English language was one of pronunciation — the Great Vowel Shift (1400–1700). This is when silent e and other spelling rules that frustrate both native and second language speakers arose. The notion of long and short vowels also changed. At one time a long vowel was one that was pronounced for a longer time than a short vowel. Take the words pan and pane. Before the Great Vowel Shift, pan was pronounced "pan" and pane was pronounced "paaaneh" with a literal looong vowel and a non-silent "eh" at the end. Being mostly a change in pronunciation, the rise of Modern English around 1550 doesn't affect our discussion of color words. Movable type printing invented in Germany around 1445 is probably more important. Books became relatively plentiful, spelling became standardized, and tracking down the first occurrence of a word became easier. The Modern English period is when the words orange and indigo were first used to identify colors.

red orange yellow green blue indigo violet

I have issues with indigo. More on that later.

There is no physical significance in color names. It's all a matter of culture and culture depends on where you live, what language you speak, and what century it is. A given wave of light has the same frequency no matter who is viewing it, but the person perceiving the color will call it a word appropriate to their culture.

Color discrimination is probably the same for all people in all cultures (all people with properly working eyeballs). Did the English see orange or violet before the French told them about it? Of course they did. They probably called orange reád (red) or geolo-reád (yellow-red) and violet hǽwen (blue) or blæc-hǽwen (dark blue) because those were the words they had available.

Why is an orange called an orange but a lemon not called a yellow and a lime not called a green?

What would you call indigo if I showed it to you? Most certainly blue. I don't know anyone who uses the word indigo in everyday conversation. Maybe some painters do. That'd be about it for indigo as far as Modern English speakers were concerned. In some languages blue and indigo are equally significant color words. Maybe the real question is do we need blue, indigo, and violet?

Frequency determines color, but when it comes to light, wavelength is the easier thing to measure. A good approximate range of wavelengths for the visible spectrum is 400 nm to 700 nm (1 nm = 10 𕒽 m) although most humans can detect light just outside that range. Since wavelength is inversely proportional to frequency the color sequence gets reversed. 400 nm is a dull violet (but violet always appears dull). 700 nm is a dull red.

Wavelength varies with the speed of light, which varies with medium. The speed of light is about 0.03% slower in air than in vacuum. If you're trying to understand color, wavelength is just as good as frequency.

We humans who speak English and live at the dawn of the 21st century have identified six wavelength bands in the electromagnetic spectrum as significant enough to warrant a designation with a special name. They are: red, orange, yellow, green, blue, and violet. Where one color ends and another begins is a matter of debate as you will see in the table below.

Wavelength ranges for monochromatic light (nm) 1 CRC Handbook of Chemistry and Physics. 1966. 2 Hazel Rossotti. Color. Princeton University Press, 1983. 3 Edwin R. Jones. Physics 153 Class Notes. University of South Carolina, 1999. 4 Deane B. Judd. Goethe's Theory of Colors. MIT Press, 1970.
color 1 2 3 4
red 647–700 647–760 630–700 620–800
orange 585–647 585–647 590–630 590–620
yellow 575–585 575–585 570–590 560–590
green 491–575 491–575 500–570 480–560
blue 424–491 424–491 450–500 450–480
violet 400–424 380–424 400–450 400–450

Which brings us to indigo. How many of you reading this learned about "Roy G. Biv" (Americans, I presume) or that "Richard of York gave battle in vain" (Britons, I presume)? Who among you learned that between blue and violet there was this special color called indigo?

Indigo. The only time I ever hear it is when my students recite the visible spectrum. Indigo is a color of relatively little importance. If indigo counts as a color then so should canary, and mauve, and puce, and brick, and teal, and so on. Where is their place in the spectrum?

How many colors are there in this swatch? How many were you taught in elementary school? The inclusion of indigo in the spectrum goes back to Isaac Newton . More on this after the data table. If you believe that indigo is an important color, then here's a set of spectral tables for you.

Wavelength ranges for monochromatic light (nm) with indigo 5 Howard L. Cohen. AST 1002 Study Guide. University of Florida, 1999–2003. 6 J.L. Morton. Color Matters, 1995–2002. 7 A Dictionary of Science. Oxford University Press, 2000. 8 Thomas Young. Theory of Light and Colours, 1802.
color 5 6 7 8
red 630–750 650–750 620–740 624–675
orange 590–630 590–640 585–575 598–624
yellow 570–590 550–580 575–858 557–598
green 490–570 490–530 500–575 515–557
blue 450–490 460–480 445–500 480–515
indigo 420–450 440–450 425–445 460–480
violet 380–420 390–430 390–425 425–460

Did Richard of York give battle in vain so that future citizens in the dismantled British Empire would forever remember indigo? Did Mr. and Mrs. Biv conceive little Roy G. so that future generations of Americans might learn the true nature of light? Where did indigo come from?

When Newton attempted to reckon up the rays of light decomposed by the prism and ventured to assign the famous number seven, he was apparently influenced by some lurking disposition towards mysticism, If any unprejudiced person will fairly repeat the experiment, he must soon be convinced that the various coloured spaces which paint the spectrum slide into each other by indefinite shadings: he may name four or five principal colors, but the subordinate spaces are evidently so multiplied as to be incapable of enumeration. The same illustrious mathematician, we can hardly doubt, was betrayed by a passion for analogy, when he imagined that the primary colours are distributed over the spectrum after the proportion of the diatonic scale of music, since those intermediate spaces have really no precise defined limits.

John Leslie, 1838

rubeus aureus flavus viridis cæruleus indicus violaceus


The human eye can distinguish something on the order of 7 to 10 million colors — that's a number greater than the number of words in the English language (the largest language on Earth).

The rods, which far outnumber the cones, respond to wavelengths in the middle portion of the spectrum of light. If you had only rods in your retina, you would see in black and white. The cones in our eyes provide us with our color vision. There are three types of cone, identified by a capital letter, each of which responds primarily to a region of the visible spectrum: L to long or red, M to medium or green, and S to shirt or blue.

The peak sensitivities are 580 nm for red (L), 540 nm for green (M), and 440 nm for blue (S). Red and green cones respond to nearly all visible wavelengths, while blue cones are insensitive to wavelengths longer than 550 nm. The total response of all three cones together peaks at 560 nm — somewhere between yellow and green in the spectrum.

  • While red, green, and blue are spaced somewhat equally across the visible spectrum, the specific sensitivities of the L, M, and S cones are not. This might seem a little confusing, especially since the L cones aren't even closely centered on the red area of the spectrum. Fortunately, the spectral sensitivity of the cones is only one part of how the brain decodes color information. Additional processing takes these sensitivities into account.

Commission internationale de l'eclairage

The relative response of the red and green cones to different colors of light are plotted on the horizontal and vertical axes, respectively. Values on the tongue shaped perimeter are for light of a single wavelength (in nanometers). Values within the curve are for light of mixed frequency. The point in the center labeled D65 corresponds to light from a blackbody radiator at 6500 K — the effective temperature of daylight at midday, a generally accepted standard value of white light.

White & black

Continuous, thermal spectra

This table is the result of an effort to interpret in terms of thermometric readings, the common expressions used in describing temperatures. It is obvious that these values are only approximations.

Handbook of Chemistry & Physics, Ninth Edition, 1922

Additive color mixing

The absence of light is darkness. Add light and human eyes to the darkness and you get color — a perception of the human visual system. The retina at the back of the human eye has three types of neurons called cones, each sensitive to a different band of wavelengths — one long, one medium, and one short. The long wavelength cones are most stimulated by light that appears red, the medium wavelength cones by light that appears green, and the short wavelength cones by light that appears blue. A monochromatic wavelength of light (or a narrow band of wavelengths) can be selected as a representative for each of these colors. These become the of a system that can be used to reproduce other colors in a process known as .

Additive primary colors
black + red = red
black + green = green
black + blue = blue

When no light or not enough light falls on the retina, the brain perceives this nothing as the color black. When the light from two or more sources falls on adjacent rods in the retina, the brain perceives the combination as a different color. The rules for combinations of the primary colors are as follows…

Additive color mixing rules
nothing = black
red + green = yellow
green + blue = cyan
blue + red = magenta
red + green + blue = white

Most of us with typical human eyes and a basic knowledge of the English language are familiar with the color yellow. This is probably not the case for cyan and magenta. Because inkjet printers (which have cyan, magenta, yellow, and black cartridges) are commonplace nowadays, it's not uncommon for people to recognize the words cyan and magenta, but not know how to pronounce them (ˈsīˌan and məˈjentə). As you'd expect given that it's a combination of blue and green light, cyan appears blue-green — something like the blue of the sky but not exactly. I'd say more like the semiprecious stone turquoise than anything else. Magenta is often confused with pink, but magenta is much more vibrant. Pink is desaturated red. Magenta is considered a pure color. (More on this later.) A close relative of magenta is fuchsia, which is a synthetic dye. I can't think of anything natural that looks like magenta.

These rules are better understood with a diagram than a series of equations.

Color mixing is not an all or nothing process. Red light and green light together appear yellow, it's true, but they can also appear orange when mixed if the red light is brighter than the green light. Red light and green light can be combined in other proportions to produce light that appears to be a color halfway between red and orange, and orange and yellow, and yellow and green. We can keep dividing and subdividing like this to produce new, distinct colors.

red red-orange orange orange-yellow yellow yellow-green green

One convenient way to represent some of the possibilities is with a continuous color wheel. Starting on the right side and going counterclockwise as is the tradition in mathematics, red is placed on the circumference at 0°, green at 120°, and blue at 240°. The complimentary colors are halfway between the primaries — yellow at 60°, cyan at 180°, and magenta at 300°. These numbers are called . White is at the origin. The distance from the origin to any point on the color wheel stated as a fraction of the radius is known as the . White is completely desaturated. Its saturation is 0%. Colors with low saturation are often identified as pale or pastel. Colors with a high saturation are bright or vibrant. Colors with 100% saturation are said to be pure.

  • optical, superposition: lamp overlap, projection TV with 3 CRTs
  • temporal, rapid alternation, persistence of vision: biased LED
  • spatial, small elements: TV/computer monitor pixels

Purple and violet are similar, though purple is closer to red. In optics, there is an important difference purple is a composite color made by combining red and blue, while violet is a spectral color, with its own wavelength on the visible spectrum of light.

Subtractive color mixing

The absence of pigment is white paper. (The absence of pigment is paper that appears white when illuminated with white light.)

Add pigment to it. (Subtract certain wavelength ranges.)

Subtractive Primary Colors
white red = cyan
white green = magenta
white blue = yellow

Subtractive Color Mixing Rules
nothing = white
cyan + magenta = blue
magenta + yellow = red
yellow + cyan = green
cyan + magenta + yellow = black

the subtractive color wheel

  • optical, superposition: paints, dyes and pigments are reflective filters
  • spatial, small elements: halftone dots

A five color press: yellow, magenta, cyan, black, spot color.

Historical junk

The painter's color wheel is a convenient way to understand how to mimic some colors by mixing red, yellow, and blue pigments. This does not make red, yellow, and blue the primary colors of the human visual system. They do not satisfy the definition of primary. They can't reproduce the widest variety of colors when combined. Cyan, magenta, and yellow have a greater chromatic range as evidenced by their ability to produce a reasonable black. No combination of red, yellow, and blue pigments will approach black as closely as do cyan, magenta, and yellow. The primary colors are red, green, and blue — not red, yellow, and blue.

Johann Wolfgang von Goethe (1749–1832), student of the arts, theatrical director, and author (Iphigenia at Taurus, Egmont, Faust). Lots of interesting descriptive information on the subjective nature of color, which many physicists of his day ignored, but does not propose a physical model of color.

The theory of colors, in particular, has suffered much, and its progress has been incalculably retarded by having been mixed up with optics generally, a science which cannot dispense with mathematics whereas the theory of colors, in strictness, may be investigated quite independently of optics.

Colour is a law of nature in relation with the sense of sight… [It] is an elementary phenomenon in nature adapted to the sense of vision…

It is not light, in an abstract sense, but a luminous image that we have to consider.

Yellow, blue, and red, may be assumed as pure elementary colors, already existing from these, violet, orange, and green, are the simplest combined results.

That all the colours mixed together produce white, is an absurdity which people have credulously been accustomed to repeat for a century, in opposition to the evidence of their senses.

Johann Wolfgang von Goethe, 1810

Now, as it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance, to the three principal colours, red, yellow, and blue, of which the undulations are related in magnitude nearly as the numbers 8, 7, and 6 and that each of the particles is capable of being put in motion less or more forcibley by undulations differing less or more from a perfect unison for instance the undulations of green light being nearly in the ratio of 6½, will affect equally the particles in unison with yellow and blue, and produce the same effect as a light composed of these two species: and each sensitive filament of the nerve may consist of three portions, one for each principal colour.

Thomas Young, 1802

Most mammals rely on scent rather than sight. Look at a dog’s eyes, for example: they’re usually on the sides of its face, not close together and forward-facing like ours. Having eyes on the side is good for creating a broad field of vision, but bad for depth perception and accurately judging distances in front. Instead of having good vision, dogs, horses, mice, antelope – in fact, most mammals generally – have long damp snouts that they use to sniff things with. It is we humans, and apes and monkeys, who are different. And, as we will see, there is something particularly unusual about our vision that requires an explanation.

Over time, perhaps as primates came to occupy more diurnal niches with lots of light to see, we somehow evolved to be less reliant on smell and more reliant on vision. We lost our wet noses and snouts, our eyes moved to the front of our faces, and closer together, which improved our ability to judge distances (developing improved stereoscopy, or binocular vision). In addition, Old World monkeys and apes (called catarrhines) evolved trichromacy: red-, green- and blue-colour vision. Most other mammals have two different types of colour photoreceptors (cones) in their eyes, but the catarrhine ancestor experienced a gene duplication, which created three different genes for colour vision. Each of these now codes for a photoreceptor that can detect different wavelengths of light: one at short wavelengths (blue), one at medium wavelengths (green), and one at long wavelengths (red). And so the story goes our ancestors evolved forward-facing eyes and trichromatic colour vision – and we’ve never looked back.

Figure 1. The spectral sensitivities of the colour cones of a honeybee. Reproduced based on Osorio & Vorobyev, 2005 Figure 2. The spectral sensitivities of the colour sensors of a digital camera. Reproduced based on original data of the Author’s.

Colour vision works by capturing light at multiple different wavelengths, and then comparing between them to determine the wavelengths being reflected from an object (its colour). A blue colour will strongly stimulate a receptor at short wavelengths, and weakly stimulate a receptor at long wavelengths, while a red colour would do the opposite. By comparing between the relative stimulation of those shortwave (blue) and longwave (red) receptors, we are able to distinguish those colours.

In order to best capture different wavelengths of light, cones should be evenly spaced across the spectrum of light visible to humans, which is about 400-700nm. When we look at the cone spacing of the honeybee (fig. 1), which is also trichromatic, we can see that even spacing is indeed the case. Similarly, digital cameras’ sensors (fig. 2) need to be nicely spaced out to capture colours. This even cone/sensor spacing gives a good spectral coverage of the available wavelengths of light, and excellent chromatic coverage. But this isn’t exactly how our own vision works.

Figure 3. The spectral sensitivities of the colour cones of a human. Reproduced based on Osorio & Vorobyev, 2005

Our own vision does not have this even spectral spacing (fig. 3). In humans and other catarrhines, the red and green cones largely overlap. This means that we prioritise distinguishing a few types of colours really well – specifically, red and green – at the expense of being able to see as many colours as we possibly might. This is peculiar. Why do we prioritise differentiating red from green?

Several explanations have been proposed. Perhaps the simplest is that this is an example of what biologists call evolutionary constraint. The gene that encodes for our green receptor, and the gene that encodes for our red receptor, evolved via a gene duplication. It’s likely that they would have originally been almost identical in their sensitivities, and perhaps there has just not been enough time, or enough evolutionary selection, for them to become different.

Another explanation emphasises the evolutionary advantages of a close red-green cone arrangement. Since it makes us particularly good at distinguishing between greenish to reddish colours – and between different shades of pinks and reds – then we might be better at identifying ripening fruits, which typically change from green to red and orange colours as they ripen. There is an abundance of evidence that this effect is real, and marked. Trichromatic humans are much better at picking out ripening fruit from green foliage than dichromatic humans (usually so-called red-green colourblind individuals). More importantly, normal trichromatic humans are much better at this task than individuals experimentally given simulated even-spaced trichromacy. In New World monkeys, where some individuals are trichromatic and some dichromatic, trichromats detect ripening fruit much quicker than dichromats, and without sniffing it to the same extent. As fruit is a critical part of the diet of many primates, fruit-detection is a plausible selection pressure, not just for the evolution of trichromacy generally, but also for our specific, unusual form of trichromacy.

A final explanation relates to social signalling. Many primate species use reddish colours, such as the bright red nose of the mandrill and the red chest patch of the gelada, in social communication. Similarly, humans indicate emotions through colour changes to our faces that relate to blood flow, being paler when we feel sick or worried, blushing when we are embarrassed, and so on. Perhaps detection of such cues and signals might be involved in the evolution of our unusual cone spacing?

Recently, my colleagues and I tested this hypothesis experimentally. We took images of the faces of rhesus monkey females, which redden when females are interested in mating. We prepared experiments in which human observers saw pairs of images of the same female, one when she was interested in mating, and one when she was not. Participants were asked to choose the mating face, but we altered how faces appeared to those participants. In some trials, human observers saw the original images, but in other trials they saw the images with a colour transformation, which mimicked what an observer would see with a different visual system.

By comparing multiple types of trichromacy and dichromacy in this way, we found that human observers performed best at this task when they saw with normal human trichromatic vision – and they performed much better with their regular vision than with trichromacy with even cone spacing (that is, without red-green cone overlap). Our results were consistent with the social signalling hypothesis: the human visual system is the best of those tested at detecting social information from the faces of other primates.

However, we tested only a necessary condition of the hypothesis, that our colour vision is better at this task than other possible vision types we might design. It might be that it is the signals themselves that evolved to exploit the wavelengths that our eyes were already sensitive to, rather than the other way round. It is also possible that multiple explanations are involved. One or more factors might be related to the origin of our cone spacing (for example, fruit-eating), while other factors might be related to the evolutionary maintenance of that spacing once it had evolved (for example, social signalling).

It is still not known exactly why humans have such strange colour vision. It could be due to foraging, social signalling, evolutionary constraint – or some other explanation. However, there are many tools to investigate the question, such as genetic sequencing of an individual’s colour vision, experimental simulation of different colour vision types combined with behavioural performance testing, and observations of wild primates that see different colours. There’s something strange about the way we see colours. We have prioritised distinguishing a few types of colours really well, at the expense of being able to see as many colours as we possibly might. One day, we hope to know why.

Bright Colors

Colors like pink or yellow are often called "bright" because of the high degree of light they reflect back. Visual light is composed of numerous different colored wavelengths which make a white light when combined. Therefore light colors such as pastel yellows or pinks are perceived that way because most light wavelengths are reflected back to our eyes. Since most light is reflected, little light (or heat) is absorbed.


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