# Of the colors we comprehend, what percentage are in light spectrum versus created in our brain?

Although all colors exist in our brain, some colors correspond to colors in the visible light spectrum (e.g. colors of the rainbow) while some are made up in our brain based on a variety of factors. For example, we could say that the green we see in the rainbow is a certain wavelength, while brown is created in our brain based on multiple frequencies. There’s no single wavelength of light which we see as brown. So for the color spectrum we see in our brain, what percent are colors which correspond to a single frequency of light versus are made up?

An analogy is with the colors we see on the computer monitor. The colors correspond to three numbers for red, blue, and green and their intensity. So the computer might have these pure color representations from 1-255 for intensity:

R-G-B
255-0-0 bright red
127-0-0 medium red
0-255-0 bright green

But the computer can also mix colors, so there are also these colors:

51-25-0 brown
255-204-255 light pink

In the computer case, I would say the pure colors correspond to a light frequency while the mixed colors represent colors created in our brain. So given that in the computer there are 255 pure reds, 255 pure greens and 255 pure blues, that would mean there are 255+255+255=765 pure colors(see note). But the total color pallet is 255255255= 16.6 million colors. So on a computer, the pure colors represent a very tiny percentage of all the colors that we see on a monitor. Is that also the same case for our brains? Do a great many colors exist only in our brain as opposed to being represented by a particular frequency of light?

(note, although the pure colors like 255-0-0 and 127-0-0 are both the same frequency for red, because the brightness is different, we consider them different colors. So although technically they are the same light frequency, I’m considering them different colors in that example.)

It’s all in our brains. There is nothing specifically ‘red’ about electromagnetic radiation that happens to have a wavelength of 650nm

And given that the spectrum is continuous (or at least so very finely discrete that it might as well be continuous), and when you mix two or more wavelengths of light to make a composite colour, you’re doing so with (in practice) infintely variable amounts, I don’t think there’s a meaningful way to express a percentage of ‘real’ vs anyting else.

(ETA: and the reason we see a mix of red+green as yellow, is because that mixture of wavelengths stimulates the receptors in our eyes in exactly the same way as pure yellow might - so actually, you could make the argument that none of it happens in our brains (unless we consider the eye to be part of the brain - some do)

Just to clarify, I don’t mean that some colors are more real than others. The red we see in our brain from the 650nm frequency is just as real as the brown we see when red and green are mixed together. But we can say that brown does not correspond to a single frequency while red does. So in a universe where there are only these two frequencies of light 650 (red) and 430 (green), if we just received 650nm light, we would see a variety of reds and 430nm of light would be a variety of greens. But if we saw both frequencies at the same time, we would see a pallet of browns which wouldn’t correspond to a single frequency of light. So if we can see 100 shades of pure red and 100 shades of pure green, then there are 200 colors which correspond to a single frequency of light. But if the frequencies are mixed, then there might be 10000 colors we see in our brain, but only 200 could be represented by a single wavelength of light.

I would say approximately one third. To simplify: there are single wavelengths that can stimulate both the blue and green receptors and single wavelengths which can stimulate both the green and red receptors, but no wavelength that can stimulate both the blue and red receptors.

I think you misapprehend the way colors work. Read up on color theory. Of course our perception of color is in our brains, but those perceptions are based on real colors out in the real world.

All the colors we can see all lie in the roughly irregular horseshoe shape of the “Spectral Locus”, closed off by the Purple line on the Chromaticity Diagram:

There are other ways to represent the colors, but they’re essentially distortions of this. Any pure spectral color lies along that curved spectral locus. If you keep making it lighter, you move inwards towards the “white point”.

So any color we can see can be represented on that chart as a “washed out” version of of a spectral line.
That includes brown.
Brown isn’t a “made up color” and it doesn’t only exist in our brains. Brown is the color of dirt and soil and animal furs and many minerals and poop. It’s as real as red or green or blue, and probably a lot more common.

So why isn’t it on the spectrum? Where is brown in a rainbow?
Answer: brown is basically really dark yellow-orange. As you lighten brown pigments, that where it tends. As you thin out poop it looks yellow* If you apply lightening procedures to the tristimulus coordinates for brown it leads you to yellow-orange.
I wrote an article about Brown for OPN a year or so ago. Trust me – any color you see you can locate on a Chromaticity diagram. Just recall that the two-dimensional plot is actually normalized so that it can fit on a 2-dimensional page. a proper chromaticity plot is three-dimensional, and includes levels of darkness. The only reason that brown doesn’t explicitly appear on the CIE diagram is because we’ve normalized it out for ease of representation.

*I know whereof I speak – I just had a colonoscopy three days ago, and spent most of Monday looking at my own thinned-out scat

Does brown exist a a single frequency like red at 650nm? I didn’t think so. Certainly brown exists as a color, but it’s not a color we perceive from a single frequency of light, is it?

If I had a photon emitter that could produce light at any single frequency, I would see X number of colors from the light. But if I had 2 devices, I would see Y colors and 3 devices would let me see Z colors. I don’t think X=Z, does it? I would think X is less than Z.

Please critique my understanding. So, at one level, the fact that any wavelength - or any combination of wavelengths - causes us to think it’s any colour at all, is an accident of the fact that we look with our eyes, and not a property of light.

And at another level, our perception that some colours come from mixing and other colours are pure is because our eyes contain three different kinds of sensors, and we sense not a continuum but sort of like a graph of three points - which our brains then perform an interpolation on, giving the impression of a continuum.

Am I close?

The colors represented by a monitor are designed to work with an 8-bit binary scale. The numbers used are rather arbitrary, and e.g. 10-bit or more monitors exist (highest I know of is 16-bit). This means that the 10-bit monitor’s bluest blue is (0, 0, 1023), but it’s not (necessarily) different then 8-bit (0, 0, 255), just that there’s more steps in between.

A monitor has a rather poor covering of all possible colors (gamut). Here’s one example of a typical monitor, the “horseshoe” is possible human-perceivable colors, the triangle is the range of this monitor.

How do you figure? There is still considerable overlap, especially in the 450-500 nm range. And even when it appears to be zero, it’s not really zero.

Shining a coloured light doesn’t represent dark colours well. Yellow-orange light is literally “bright brown” - we just don’t call it that.

Read what I wrote – yes, brown does exist as a single frequency. Actually, since there is a range o browns, there is a range of frequencies, going from yellow to orange on the spectral Locus. It’s just a very dark yellow-to-orange.
Certainly you can represent brown by a mixture of colors, but that’s the nature of our tristimulus eye response – we can represent ANY color that falls within the gamut of your three base colors as a mixture of the three. But I can also take the point represented by the color on the CIE Chromaticity diagram and draw a line radiating outward from the “White” point through that point that will eventually strike the locus at the wavelength (and frequency, which is inversely proportional to that wavelength) corresponding to that color. And it’s as true for brown as for any other color.
Here are some people that agree with me:

http://www.mat.univie.ac.at/~kriegl/Skripten/CG/node9.html

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/cie-1931-color-space

Sorry – it just occurred to me that something I said isn’t quite correct.

But it’s not about Brown – I stand by everything I’ve said about Brown.
But there is one class of color that you can’t represent by a single wavelength or frequency, regardless of how light or dark you make it. That’s purple. Purple isn’t a spectral color (although Violet is – the two are distinct colors). Purple is defined as a color lying along that straight section at the bottom of the Chromaticity diagram that joins the Blue/Violet end of the Spectral Locus (the curved part) to the Red End. That’s the Purple Line, and to create something of that color you have to mix some color from the Red end of the spectrum with something from the Blue/Violet end*. If you choose any point in the purple part of the Color Diagram and project a line from the White Point at the center you won’t eventually hit a point on the Spectral Locus; you’ll hit a point on the Purple Line, and to make a color on the Purple Line you have to mix Extreme Red and Extreme Violet in some proportion.
So there are, indeed, some colors that do not exist as pure spectral colors, or which can’t be represented by a single wavelength. But they’re all shapes of purple.

…and now you know why.

*My mother always said I’d come to a Violet End.

I hasten to add that the color “isn’t in your brain” – it’s definitely in Nature. but it can’t be represented by a single wavelength.
Just a By the Way – our brains perceive three degrees of freedom in color – that’s that whole “tristimulus” thing, and explains why we can get away with three-color printing and three colors on our monitors. we can perceive the entire spectrum with three degrees by mixing different proportions of the stimuli. If that’s what you mean, then So Be It – any color perception is going to require a mix. the spectrum has an infinite number of shades of color, but we don’t need an infinite number of stimulus responders to see them. We mix the response in our brains.

This is why the Mantis Shrimp has such a complex vision system with more than three degrees of freedom – it’s brain isn’t big or complex enough to do the color mixing there. it has to perceive them directly.
But a word of warning – as soon as you think you have color vision theory worked out as a three degree of freedom thing, someone will spring Edwin Land’s Retinex Theory of Color ion you, and ruin everything. Because Tristimulus alone doesn’t explain the Land Effect.

Call me silly, but I don’t get this post.

Is the Colour Diagram explaining independent features of nature, or explaining vision?

To put it another way, if an imaginary machine from another universe analyzed our light, would something like the Colour Diagram be a result?

Forget brown for a moment, because it’s too easy to argue that it’s really just dark yellow.

White light does not exist as a point on the spectrum - it only exists as a perception of a mixture.

I still don’t know how to answer your question about percentages though. I am inclined to say that there are actually zero pure colours - because even when we are looking at a pure, single wavelength source of light that corresponds with the peak of receptivity of one specific flavour of retinal cone cells, the other types of cone cells are still getting stimulated - only not so much - and it’s still our brain that filters out and interprets the meaning.

I think we’re actually not capable of truly sensing any pure spectral colour, as a pure sensation.

Complex input (everything we can look at)

(multiplied by)

Complex receiving apparatus (eye)

(multiplied by)

Complex interpretation apparatus (brain)

(multiplied by)

Complex output apparatus (brain again, explaining)

(multiplied by)

Complex re-interpretation apparatus (other brain, reading the explanation)

(equals)

One big mess.

Land’s experiments clearly demonstrate that the perception of color isn’t bound by frequencies. The perception of color is a decision made by the brain; it’s not a slave to frequencies.

Winding back. If you look at the CIE colour diagram, the edge of long arc enclosing the top of the diagram corresponds to spectrally pure wavelengths. Everything else is a mixture, especially the straight edge along the bottom. The entire enclosed area, not including the arc, is a tristimulus mixture colour.

So in a mathematically correct sense, there are infinitely more “unreal” colours than real ones. But of you want to be more correct in terms of perceptual reality, the limits of our eyes colour resolution means that there is a thickness to the edge, and colours just within the boundary are indistinguishable from the edge. That would get us a ratio of areas. (One would need to crunch some numbers to work it out.) Complicating matters the colour resolution is better towards the red end, so the calculation becomes harder, eventually one would need to work out the number of discrete colours in both regions. Further complicating matters, the CIE diagram is actually only the base of the 3D volume, so the full answer is a ratio of enclosed volume elements in two regions. (The 3D volume is not a simple vertical extension, it changes shape as it goes.)

I’m sure someone has already done the calculation. I need to be on the road in a few minutes so I’m not going to try to find it.

Is this the same Mr. Land who gets part of the credit for the way the classic Polaroid camera works?

Yes.