[braindeath]

> If these are the primary colors, why aren't they what printers use? Printers use cyan, magenta, and yellow.

There really is no such thing as "the" primary colors. The school primary colors are still primary as those used by a printer, and are based on historically widely used pigments. CMY (and usually K) allow for a wider gamut. But even this isn't perfect. There are printing processes with more primaries to get a wider gamut.

> And the only thing that makes these colors primary to us is that they're the colors that the cones in our eyes perceive.

This isn't quite right. For one, our cones are not monochromatic receptors, and moreover, they overlap! There isn't really just one true red, green, blue used in computer monitors either.

Because of the way our brain perceives colors (metamerism), you can create a wide gamut of colors with "alternative" primaries.

[tiglionabbit]

What sort of alternative primaries? Just slightly-adjusted colors that still trigger each of the three cones individually?

[braindeath]

You can't trigger cones individually. The cones are responsive to a wide-spectrum and they overlap, especially in the case of the L and M receptors. The peak wavelength of the L receptor (the "reddest") is about 580nm -- that is not red, that's yellow-green.

Color vision and stimulus is not a straightforward mapping of primaries triggering cones. If it were that simple you could trivially render all perceivable colors with 3 chosen primary colors. This is impossible to do. You can mathematically define 3 primaries that cover the entire visible spectrum but they cannot physically exist (complete, but imaginary). Any chosen set of 3 primaries is a compromise. For subtractive materials it is trickier, which is why photo inkjet printers will use up to 8 primaries.

You might find this informative: https://web.archive.org/web/20080717034228/http://www.handpr.... The section starting with Maxwell and the "3 artist's misconceptions" especially.

But the only general definition of primary is basically just any set of colorants that can be mixed to get a useful gamut. In subtractive materials, this is why you won't see a painter messing around with mixing cyan, magenta, and yellow (better explained in the link).