Colours would be visible fundamental wavelengths/frequencies. Not harmonics of the fundamental.
We’re not talking about harmonics in the recorded music, but harmonics added where they did not exist at all. Adding harmonics to piano, that are different than piano overtones, is distorting piano sound (creating beats). You might like it, but it is distorted sound with reduced clarity. Trying to cover bright metal dome tweeter with warm sounding gear is adding further reduction in clarity. Now, we have both odd and even harmonics added, when replacing tweeter (or speakers) seems to be the better choice, at least to me.