is the rainbow as rich as the painter's palette?

[aphar]

Does every color we see correspond to a fixed wavelength?

I.e., is it true that for any mixture of colors we see there exists a wavelength so that the corresponding color is indistiguishable from the mixture?

No: our color vision is 2 dimensional: 3(RGB) - 1(luminosity) = 2, while the wavelength offers only one dimension, therefore there must be many colors on the palette that are not visible in the rainbow

E.g.: brown.