Comment by neurosphere

Painters don't stick to just three pigments, but all the pigments they use CAN be created by mixing three primaries in different proportions. I'm quite sure that's the whole basis for Thomas Young's colour mixing experiments performed in the 1800s, finding that every colour we see can be made with mixtures of three lights. Any more than that is redundant. Similarly, while an RGB pixel on your monitor may not emit all single-wavelength light, it doesn't negate the validity of the statement that three single wavelengths can be used to recreate all colours visible to the human eye. I'm quite sure that some RGB pixels are indeed single wavelength, though I would love to hear from those of you who know better about this because I don't know for sure...