Slightly biological, hopefully physical enough to be answered.

Suppose a magenta hue is represented by a mix of red and blue pigment. This is all very well for a creature with red and blue photoreceptors, but suppose it was seen by a creature which had a magenta-sensitive receptor, but no red or blue. Would the colour appear the same (insomuch as a qualititave concept can appear the same, I suppose I mean 'Would they interpret it as colour of the same wavelength?')?

The crux of my question is, do the particular bands in which photoreceptors are activated affect vision of additive, as opposed to pure, hues?

Finally, completely off-topic, but if there happens to be a biologist around, do animals on the whole have similar photoreceptors, or are they placed largely randomly?

Thanks, Wyatt

  • $\begingroup$ I believe this belongs in a biology related messageboard. $\endgroup$
    – Cem
    Dec 16, 2010 at 10:07
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    $\begingroup$ @Cem: we had a course on color science that was called "Applied Physical Chemistry", don't ask me why :) $\endgroup$
    – gigacyan
    Dec 16, 2010 at 10:21
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    $\begingroup$ The "physical" color is given by the spectrum. The human eye only measure a part of it, as well explained in @gigacyan's answer. You can have two different miwture which could appear the same to human eye, but with different spectra, and then would appear different to some other animal species. This is called metamerism : see en.wikipedia.org/wiki/Metamerism_%28color%29 . $\endgroup$ Dec 16, 2010 at 10:44
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    $\begingroup$ You won't get a magenta hue by mixing red and blue pigment. You get it by mixing red and blue light. It's a little unclear what you mean physically by "pure hues". Is that supposed to mean light with a single frequency? $\endgroup$ Dec 16, 2010 at 13:34
  • $\begingroup$ @Frédéric Grosshans The page on metamerism is precisely what I was looking for, thank you. And kudos on interpreting my rather bumbling quesiton. $\endgroup$
    – wyatt
    Dec 19, 2010 at 12:44

1 Answer 1


No, they will not appear the same. Humans have three color receptors so any possible color for us is just three numbers in RGB space. However, electromagnetic spectrum is continuous and there is an infinite number of spectra that would produce the same RGB stimulus. That is why you perceive the this page as white although it is in fact a combination of R,G,B tuned for a human eye. A creature with other set of photoreceptors would not see this page as white. Actually, white is also subjective, see how many settings for white balance your digital camera has, but suppose that we set it to 'daylight' and consider the continuous spectrum of sun as white.

Another interesting point: magenta hue cannot be represented by any single wavelength, check out this famous horseshoe diagram:

CIE 1931 Color Space

So magenta is even more subjective than green or red.

Generally, most some animals don't perceive colors at all. There are some who have only two receptors and others who have four. Mantis shrimp has 16 different photoreceptors!

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    $\begingroup$ +1 if you explain what the x and y axis represent in the diagram :-) $\endgroup$
    – Sklivvz
    Dec 16, 2010 at 14:38
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    $\begingroup$ I want to note, that the human eye has three different color receptors, each with its own frequency response, so for humans color is a three dimensional object (strength of the stimulation for each type being the coordinates). If we had a fourth type of receptor color would be a four dimensional object. For some color blind people color is only two dimensional or less. $\endgroup$ Dec 16, 2010 at 15:19
  • $\begingroup$ @Sklivvz: See en.wikipedia.org/wiki/CIE_1931_color_space $\endgroup$
    – kennytm
    Dec 16, 2010 at 19:13
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    $\begingroup$ WARNING:This is philosophy :) Qualia ---but the concept was supported by physicists like Schrodinger --"The sensation of colour cannot be accounted for by the physicist's objective picture of light-waves. Could the physiologist account for it, if he had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain? I do not think so." From "What is Life" $\endgroup$
    – Gordon
    Feb 4, 2011 at 7:39
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    $\begingroup$ @Gordon: Thank you for the correction, I changed the part about "most animals". As for individual color perception - people have actually found the way to compare it. The way how CIE 1931 color space was defined is that subjects were presented different colors and had to match them with a combination of RGB primaries. And if you are interested what different people call "red" - just make a large survey, like this one: blog.xkcd.com/2010/05/03/color-survey-results $\endgroup$
    – gigacyan
    Feb 4, 2011 at 8:07

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