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Normally, the human retina contains four types of light-sensitive receptors (opsins): three types of cones and one type of rods. Receptors contain proteins-chromoproteins - iodopsin in rods, rhodopsin in cones. The role of the latter in bright light is insignificant, therefore for a person there are three "basic" colors: blue, red, green - all the shades we perceive are formed by their combinations. And what would the world look like if there were not three such colors, but four? (Tetrachromacy is the perception of the visible range of electromagnetic radiation by combinations of four primary colors. The eyes of tetrachromats contain four types of light receptors with different degrees of perception of different subranges of the visible spectrum) The painting "Rainbow Eucalyptus" by the Californian artist Conchetta Antico, who has functional tetrachromacy, makes it possible to appreciate the variety of colors, perceived by people with four-color vision. On the left, for comparison, is a photograph of the landscape shown in the painting.

Many insects, some fish, and most reptiles and birds have four-color vision. The extra pigments allow these animals to see in the ultraviolet range. In humans, tetrachromacy occurs only as a rare genetic abnormality. It does not affect the width of the perceived part of the spectrum, but it significantly increases the sensitivity to shades. However, by the standards of mammals, humans have excellent color vision: many mammals have two-color vision, if not even monochrome. This regression compared to the evolutionary precursors of reptiles was most likely associated with the nocturnal lifestyle of early mammals. In the dark, the effectiveness of color vision drops sharply, and the loss of two types of cones "went unnoticed." As a result, primitive animals retained only two types of receptors - red and ultraviolet. Later, when mammals "came to light" again, some groups were able to restore tricolor vision. For primates, many of whom feed on fruits, this vision is very useful: it allows you to detect brightly colored fruits among green foliage, as well as determine their ripeness. The receptor that perceives the green color arose as a result of a duplication of the “red receptor” gene and subsequent mutation, which shifted its sensitivity to the short-wave region. But the receptor for ultraviolet light for human ancestors has become useless: their lens does not transmit the corresponding wavelengths. But on the basis of this receptor, as a result of a series of mutations, a receptor for blue light arose. Such mutations, which alter the peak of the spectral sensitivity of photoreceptors, can also endow their carriers with four-color vision. However, much more often they make one or another iodopsin non-functional: as a result, dichromacy occurs - color blindness. The genes for "red" and "green" iodopsins are located on the X chromosome, which is present in two copies in the chromosome set of women and only one copy in men. That is why color blindness is predominantly a male ailment: in women, due to the presence of a "reserve" X chromosome, it develops extremely rarely. For the same reason, only women can become tetrachromats: this requires that one of the X chromosomes contains a normal copy of the gene, and the other contains a mutant gene encoding a protein with a shifted photosensitivity peak. Since each of the iodopsins makes it possible to differentiate about a hundred shades, a person with normal vision can potentially distinguish about a million color combinations. The addition of another type of receptor increases this number to one hundred million. Concetta Antico is a carrier of a mutation in the gene of "red" iodopsin, the sensitivity of which has shifted to the short-wave region. Special opportunities are best manifested when distinguishing between reddish-yellowish and violet shades: in the color scheme of her paintings, the emphasis is on these tones.

And here we return to my question: how much will the color perception of my genetically modified people change if I give more than four photoreceptors (5, 6, etc.)?

If the spectrum contains seven primary colors (red, orange, yellow, green, cyan, blue and violet), then if we add each photoreceptor to these colors, we will perceive many more shades?

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  • $\begingroup$ haven't you already answered your own question? It gives an increase in differentiation of colours. If the visual cortex is keeping up with the extra colour identification, you'll probably get some more art and colour coding like traffic signs. Extra insta filters and such. Is such list wahat you require as an answer? $\endgroup$ – Trioxidane Nov 7 '20 at 6:37
  • $\begingroup$ The question means how much will human vision change if instead of three we have more than four photoreceptors? I am also interested in what these photoreceptors would represent biologically, in what ranges of visible radiation they would be located for greater intensity. So the human eye has the highest sensitivity to light in the 555 nm (540 THz) region, in the green part of the spectrum. $\endgroup$ – French Thompson Nov 7 '20 at 7:39
  • $\begingroup$ Related $\endgroup$ – L.Dutch - Reinstate Monica Nov 7 '20 at 8:12
  • $\begingroup$ Just to be clear you're suggesting a scenario where the brain has evolved seeing with three and you're adding new ones now, right? $\endgroup$ – Lio Elbammalf Nov 7 '20 at 9:07
  • $\begingroup$ These are artificially created people, evolution is not at all worth it, so that they do not lose something else. $\endgroup$ – French Thompson Nov 7 '20 at 10:14
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Color is a sensation. It exists in the mind. Color is not a physical quantity; it does not exist in nature.

A phrase such as "sunflowers are yellow" has meaning only for a normal-ish member of one species. It is meaningless to compare color perception between species with different visual receptors.

For example, here is one of a rather famous series of paintings by Vincent van Gogh:

Van Gogh, Sunflowers (1889)

Vincent van Gogh, Sunflowers, 1889. Photographic reproduction of a painting in the Amsterdam Van Gogh Museum; available from Wikimedia; public domain.

The sunflowers are yellow. The background is yellow. And yet, were one to examine the spectrum of the light emitted by the computer monitor displaying this picture, one would most likely notice the total absence of the wavelengths which produce the color yellow in a rainbow. Which brings about the first observation about color as perceived by humans:

For extended objects, i.e., objects which occupy a significant part of the visual field, color is determined by the power spectrum of the light emitted or reflected by those objects, but the relationship is not one-to-one. An infinite number of very different combinations of wavelengths can produce the same color. But, one specific wavelength in isolation will always produce one and only one color.

(For small objects, i.e., objects which cover only a small part of the visual field, there is no direct relationship between perceived color and the spectrum of light. The perception of color of small objects is a very complicated subject, as it depends enormously on the color of nearby objects; the human mind behaves as if it had some sort of "color sharpening" filter, which makes it impossible to predict the color perceived for small objects from the spectrum of light coming from those objects.)

For extended objects, the human mental system responsible for the perception of color behaves as if it had three inputs:

  • A quantity called luminosity, which is roughly what is reproduced by a black-and-white photograph.

  • A quantity on the axis yellow–blue with saturated yellow at one end, saturated blue at the other end, and completely unsaturated gray in the middle.

  • A quantity on the axis red–green, with saturated red at one end, saturated green at the other end, and completely unsaturated gray in the middle.

(This is why we can imagine and understand reddish-yellow, yellowish-green, and greenish-blue, but we cannot imagine and cannot comprehend yellowish-blue or greenish-red.)

Note that I said as if it had three inputs. Color is a sensation which exists in the mind. It has no physical reality. Those as if signals do not exist as physical quantities anywhere in the physical realm; they are as metaphysical as the mind which perceives the color.

The good thing is, we can predict the color perceived by a "standard observer" given the power spectrum of the light emitted or reflected by an extended object. This allows us to design various schemes of color reproduction, which, within the limits inherent in every such scheme, allow for predictable color sensations in the mind of said standard observer.

But only in the mind of a standard observer.

The reproduction of Van Gogh's picture shown above looks very much like the original Van Gogh picture when observed by a human standard observer. It would look very different from the original Van Gogh picture when observed by an observer who is not a standard human observer. Such as a bird, or a bee, for example. For a human standard observer, the sunflowes in the reproduction are yellow. For a bird, or a bee, they would not have the same color as the sunflowers in the original painting; what color they would have we cannot say, because it is meaningless to compare color perception between species with different visual receptors.

When we say that a standard human observer is a trichromat, what we mean is that the standard observer can color match a given source of light by manipulating the intensity of three different monochromatic sources.

And here we come to the crux of the problem, namely the distinction between functional physiological tetrachromaticity and functional mental tetrachromaticity.

Since I cannot show colors as would be perceived by a hypothetical human with functional mental tetrachromaticity, we'll have to make do with ordinary trichromaticity.

Look at the photograph shown below, which depicts a display of colorful umbrellas, which at some point in the summer of 2020 graced AFI Palace, a large shopping mall in Bucharest.

Colorful umbrellas, showing three different perceptions of color

Colorful umbrellas, displayed at some time as a decorative element in AFI Palace, Bucharest. Own work, available on Flickr under the Creative Commons Attribution license.

On the left, the umbrellas as seen by a physiological and mental dichromat. In the middle, the umbrellas as seen by a physiological trichromat whose mind is still operating in dichromat mode. On the right, the image as seen by a physiological and mental trichromat.

  • Looking at the image as perceived by a physiological and mental dichromat, we notice that what we see as red, orange and yellow umbrellas are all equally red; and green umbrellas are indistinguishable from the lavender ones.

  • Looking at the image as perceived by a physiological trichromat whose mind still works in dichromat mode, we notice that the red, orange and yellow umbrellas are now perceived as different shades of red, and green umbrellas begin to be distinguishable from the lavender ones.

  • And finally, the image perceived by a functional physiological and mental trichromat contains color which the dichromatic mind could not even imagine.

Which concludes the experimental part of the answer; based on which I can confidently say that:

  • A human possessing full physiological and mental tetrachromaticity would see the world in a very different way from what is seen by the ordinary human trichromatic standard observer.

  • The differences would be dramatic, with objects which appear of the same color to a standard trichromatic observer gaining strikingly different colors for the hypothetical human physiological and mental tetrachromat.

  • Our trichromatic mind cannot even imagine the colors seen by a fully functional physiological and mental tetrachromat.

  • On the other hand, a human observer who is a functional physiological tetrachromat, but whose mind operates the same as the usual human standard trichromatic observer, will see the same colors we ordinary humans see, but they will be able to distinguish between objects which appear of the same color to us.

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  • $\begingroup$ Actually thanks to brain plasticity a human tetrachromat should actually perceive differently, and this has withstood testing. you brain actually has to learn to see color, for most people this happens while you are an infant. you can even trick your brain in to seeing new colors or seeing colors drastically different. $\endgroup$ – John Nov 7 '20 at 17:19
  • $\begingroup$ @Jonh: Yes, you are right. I even hinted at this, with the physiologically functional trichromat whose mind "still works in dichromat mode". But I cannot think of a way of exploring those aspects in a short(ish) answer. If you can, please do so; I will upvote it. $\endgroup$ – AlexP Nov 7 '20 at 17:34
  • $\begingroup$ So is the short answer to the actually asked question "You can expand it infinitely as long as you expand the mental processing as well?" $\endgroup$ – Marky Nov 10 '20 at 23:58
  • $\begingroup$ @Marky: With the observation that you are sacrificing visual acuity... In practice, humans tend to consider that acuity is more important than color discrimination. $\endgroup$ – AlexP Nov 11 '20 at 0:05
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There are documented cases of (very) rare individuals who can just see into the near/lower ultraviolet (blue end of the spectrum) and therefore perceive 'colors' differently to the rest of humanity. So if the genetics involved was characterized and copied I suppose people could be engineered with that characteristic. Beyond that?

Other frequencies? would require a total reorganization/restructuring of the human eye and brain or more likely multiple sets of 'eyes' designed each designed to detect a different part of the spectrum.

It should also be remembered that evolution has adapted our eyes (and other creatures) to take maximum advantage of the light frequencies hitting the Earth at ground level. That's about 44% of the relevant output, with that output trailing off (again as a % of the total) at the higher and lower edges of the spectrum. So we already see a large chunk of the useful energy being emitted.

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There is essentially no limit to how many colors you could perceive by adding more kinds of photoreceptor.

Say for argument's sake that we can perceive ten different levels of intensity in each waveband; then with three primary colors, you can see a thousand (10x10x10) distinct colors. Suppose that you add a third primary color, say a "yellow" wavelength between green and red. You would now be able to see ten thousand distinct colors.

But it is important to be clear that this is not simply about being able to resolve more in-between shades; with four primary colors, you will perceive differences between things that are exactly the same color to other people. In other words, you will see entirely new colors.

So, as you increase the number of photoreceptors, you exponentially increase the number of colors there are to distinguish between. If you have a hundred different photoreceptors, your eyes are basically spectrometers, and every molecule has its own entirely unique color (somewhat like how smell works).

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