So what is called "Cyan" on a computer screen (#00ffff) is as close as you can get to the real color by mixing RGB.
In real life, Cyan would be a pure lightsource at, say, 490nm. When that light hits your retina, it will stimulate the S, M, and L receptors in different ways, and your brain will do the math on those differential signals to "rebuild" the color.
RGB is a hack we've come up with to stimulate each of the cones separately. But it cannot cover the whole gamut. Look at this diagram: [0]. The triangle is the gamut that results from choosing a particular set of 3 primaries. Only colors inside that triangle can be simulated by the mixing of those primaries. Colors that lie outside of the triangle cannot be created.
There are other gamuts that cover more visible colors, like the Adobe Wide Gamut [1]. Even this one still manages only 78% of the colors we're capable of seeing. And still no pure Cyan.
The thing we call "cyan" on our computer screens is desaturated. The mixing of the green and blue primaries that mimics pure 490nm unfortunately also stimulates the L cone too much. From [2]:
> No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 color space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
Kind of like our music system isn't actually harmonic, we've just learned to accept the small error (distributed evenly across notes, called equal temperament). But you try telling the masses they're actually listening to garbage, or getting musicians into just intonation and mathematically pure harmonics. It's impractical.
Similarly I fail to see the success in debating how truly cyan this or that tech can produce. It doesn't seem to invalidate my point of RGB being an old "invention" (of evolutionary process).
#00ffff is cyan. The fact that almost all devices fail to reproduce that color exactly is an implementation detail.
Fun trick, you can oversaturate your receptors by intensely staring at max red for a while, then close your eyes, and my understanding is that this perceptual illusion lets one briefly observe pure cyan.
In real life, Cyan would be a pure lightsource at, say, 490nm. When that light hits your retina, it will stimulate the S, M, and L receptors in different ways, and your brain will do the math on those differential signals to "rebuild" the color.
RGB is a hack we've come up with to stimulate each of the cones separately. But it cannot cover the whole gamut. Look at this diagram: [0]. The triangle is the gamut that results from choosing a particular set of 3 primaries. Only colors inside that triangle can be simulated by the mixing of those primaries. Colors that lie outside of the triangle cannot be created.
There are other gamuts that cover more visible colors, like the Adobe Wide Gamut [1]. Even this one still manages only 78% of the colors we're capable of seeing. And still no pure Cyan.
The thing we call "cyan" on our computer screens is desaturated. The mixing of the green and blue primaries that mimics pure 490nm unfortunately also stimulates the L cone too much. From [2]:
> No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 color space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
[0] https://commons.wikimedia.org/wiki/File:CIE_chromaticity_dia...
[1] https://en.wikipedia.org/wiki/Wide-gamut_RGB_color_space
[2] https://en.wikipedia.org/wiki/Color#Color_in_the_eye