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There are multiple levels of processing here, so this pure trichromatic model turns out to be a somewhat misleading mental model to fix on. The signals from three types of cone cells are processed immediately within the eye into 3 different signals, a lightness response (L + M), a yellow-blue response (L + M – S), and a red-green response (L – M), before those combined signals go through further processing. LMS signals per se don't get sent anywhere further up the chain. If you are red-green colorblind that means you aren't getting much if any useful signal from the L – M response, either because your 'L' and 'M' cones are too close together to meaningfully distinguish, or because you are missing one of the two entirely.

[Calling the LMS cone types Short = 'blue', Medium = 'green', and Long = 'red' creates a lot of confusion. Stick to long/medium/short labels. A statement like 'there is no such thing as pure green' is nonsensical. As Newton figured out, light rays themselves are not colored; color is a higher-level percept constructed in the human brain. Green is no more or less 'pure' than blue or orange or purple.

For that matter calling the RGB primaries 'red', 'green', and 'blue' causes some confusion also, as these primaries are seen as an orangish red, a yellowish green, and a purplish blue, nowhere near the 'unique hues' red, green, or blue. E.g. the use of these names in named CSS colors, etc. is a horrible human interface.]

If you are interested in the details, this is one of the better sources online http://handprint.com/HP/WCL/color2.html [I would refer you to books but those are hard to accessibly hyperlink].

As for rods, their response is pretty much ignored during the day. They are much more sensitive, so start kicking in when the visual system adapts to very low levels of light where the cone cells aren't getting much useful signal.

The article under discussion in this thread is full of technicalities, and in a brief skim I didn't notice any obvious errors, but it ignores several of the most important high-level features of human color perception – and gets a bit confused about the relationship between color reproduction via additive RGB displays, color measurement by instruments, and models of human vision – and therefore isn't the most useful resource for someone new to the subject, IMO.

Which book(s) would you recommend?

Agreed, it's a lot of different sources of information. Color perception and reproduction are deep subjects and many of the results/assumptions are in turn subjective (e.g. dependent on the person). However, this is a lot better than most of the simple presentations I've seen on-line.

One of the noticeable gaps in the article was a discussion of white-point or color temperature even though it starts to discuss illumination. It's critical to realize that our perception of color is influenced by the surroundings and relative to what we accept as white. That's part of why when you're out in the snow with rose glasses on things start to look normal, until you take them off... also why people argue about the blue-black or gold-white dress.

These sorts of mental models are pure, pure gold. I spent a lot of time looking at various descriptions of color spaces and while I could do all the calculations, I never had a clear understanding of _what_ I was doing and _why_ it worked.

So in 4 paragraphs you've managed to leapfrog my understanding far more than weeks of formal training.

> Additionally there is no such thing as pure green, because all radiation that green feeds on will also feed either red or blue as well.

You can exploit cone fatigue to experience a greener green: make the red and blue people play tug of war by themselves with high intensity, then if you immediately call in green's fresh strength, red and blue will be easily overpowered.

(Slight model inaccuracy: the cells themselves aren't the colors, they're just cells with low-, medium-, and high- frequency sensitivity. Interestingly, the color experience you get from activating just one type of cell is different from the one you get from monochromatic light at their respective spectral peaks. I find deep (long-wavelength) red, for example, to be rather different.)

As a "colorblind" person, this is a bit frustrating so bear with me.

Few people are literally "colorblind". You don't have one cone cell of each type, you have loads of them. Colorblind people generally have fewer of them (or some of them are shifted) but usually only for one type (usually red/green).

The "red" and "green" cones overlap quite a lot[1] and they also overlap with the rods. This means that even if you lack some "green" cones you will still likely perceive "green" colors, except they will be hue-shifted into red and less saturated. Simulating this effect for people with normal vision usually results in "muddier" colors.

Your analogy is pretty good otherwise (except as someone else pointed out, that's apparently not at all how the signals are processed) but this seems to be a widespread misconception about color "blindness", so I felt it's important to point that out.

It's also worth mentioning that it's meaningless to say "there's no such thing as pure green". Physically isolating certain wavelengths produces a "pure green" light. What someone with normal color vision perceives when they see "pure green" light is the result of multiple cones being stimulated at the same time, sure, but there's no one-to-one mapping of cones and colors except for some very small bits at both ends of the spectrum, which is why the cones are generally referred to as L/M/S (for long/medium/short wavelengths) rather than R/G/B (for the colors).

A tetrachromatic person (assuming the fourth cone type's peak sits somewhere between 420nm and 564nm, i.e. "blue" and "red") likely won't enable someone to perceive colors outside that range. In your example, the 3 people are forming a lopsided triangle. Adding a fourth person makes it a four-sided polygon but doesn't add another dimension. Tetrachromacy (depending on where the cones lie) would make it easier to distinguish certain shades though by making them more vibrant -- to them, normal vision would appear as "muddy" as my vision does to someone with normal vision.

[1]: https://commons.wikimedia.org/wiki/File:Cone-response-en.svg

EDIT: Because some people find that interesting, here's a page with a bunch of photos that fairly accurately simulate my color vision (i.e. looking randomly at both sets of photos I wouldn't be able to tell you which one is the original): https://web.archive.org/web/20160308043950/http://critiquewa... -- apparently brown skin looks green to me.

> Physically isolating certain wavelengths produces a "pure green" light.

What, can we get an wavelength-energy distribution that looks like a Dirac delta?

There are discontinuities in physics but you kinda need to be careful to see if what you think is a discontinuity is one.

You can use a laser. https://en.wikipedia.org/wiki/Coherence_(physics)#Temporal_c...

What this is saying is that a perfect sinusoid has a single frequency?

Of course. Sorry, I'm stupid. Born like that.

The genes for L and M cones are on the X chromosome and there are some variants for each type, or sometimes one is missing. This mostly affects men, since they have only one X chromosome, so if the gene for e.g. the L cones is missing then they end up as dichromate or if they get two variants which overlap too much there isn't much differential signal. A small number of women end up as tetrachromats because they have e.g. 2 types of L cones with slightly different spectral sensitivity, one from each of their X chromosomes. It's still not clear as far as I know whether such women see a dramatically wider range of colors, or whether it's more of a slight advantage in discriminating some near-looking colors which appear identical to other viewers. Human tetrachromacy is rare and hasn't been studied too deeply from what I've seen.

FWIW from the few studies about human tetrachromacy that I've heard of it seems that the women do in fact just see some similar-looking colors as more obviously distinct.

AIUI In order to see any "new" colors (i.e. outside the "visible" spectrum) they'd need to be more sensitive to UV or IR light (which in addition to being able to perceive those in isolation would presumably also mean being able to perceive those in combination, which indeed would drastically extend the numbers of colors).