[logicrook]

>Painters don't stick to just three pigments, but all the pigments they use CAN be created by mixing three primaries in different proportions.

No. Paint is not light, and has physical properties that constrain what you can do with them. Some colors just don't mix well together, so you'll find recommendation on what exact type of pigment work best for specific mixes.

[neurosphere]

Yeah, that's true, paint is subtractive, as opposed to light. But isn't it still the case that all pigments can be achieved by mixing three primaries? Whereas in light mixing the primaries are considered to be red, green, blue, in pigment mixing they are considered to be cyan, magenta, yellow. Obviously please correct me if I'm wrong (no sarcasm implied), links would be useful.

[rspeer]

No. No matter whether you're doing additive or subtractive color, there will be perceivable colors that you can't reach just by mixing a finite number of primaries. In fact, CMYK printing has a rather small gamut, with more colors that are clearly unreachable than RGB has.

Perceivable colors are a three-dimensional space, and colors you can make out of three primaries are a three-dimensional space, but they're different shapes. Mixing primary colors is a way of interpolating between them linearly, and the space of perceivable colors doesn't have convenient linear edges.

Here's an example. Your printer has cyan, magenta, and yellow ink (and also black for convenience). So it can print any color, right? It should be able to print bright lime green, something that looks like RGB #00ff00, right?

No, in fact, you can't print that color without "spot color" ink. When you mix cyan and yellow to make green, it will necessarily get darker. If you use enough cyan and yellow to get full-saturation green, it'll be too dark. If you use less ink, it won't be saturated enough.

Lime green is outside of the CMYK gamut. Similarly, the deep cyan you'd get by printing with lots of cyan ink is outside of the RGB gamut.

[logicrook]

No, I meant some pigments won't mix well because of other physical properties. The only example I can think of from the top of my head is in the Quiller, Color choices. It is mainly concerned with watercolors, and some colors mix uniformly (so blue+yellow -> green) and some look like an emulsion (so not green, but yellow with blue bubbles inside). Which can be interesting, because the mixing of colors is thus done by the eye instead of the paint, and is in fact one of the major painting effects used by impressionists.