The colors don't necessarily look identical to him. If you have trouble see in green, and two colors differ by how much green is in them, you might perceive the two colors to be closer together than someone with normal color vision perceives the two colors.

I think that's what you see in a lot of the pictures. Look at the green in the coat in the self-portrait. What appears as distinct lines of green in the normal color version becomes muted in the modified color version and blends in much more with the other colors, but with just enough difference to lend a rich texture to the coat.