What color would a proton be if it were visible to the human eye?

If a photon hits a proton, would it have a color? What color would it be?

Answer

Blue.

The proton is way smaller than a wavelength of visible light. But blue light has a shorter wavelength than any other visible color, red light is longer wavelength, blue is shorter, other colors in the middle somewhere.

White light is a mixture of all the colors of light, all the wavelengths in the visible range. If you illuminate the proton with white light, almost all the white light will just go past the proton, not reflect back, because the proton is so small. But of the small amount of light that does reflect back, a higher fraction of it will be blue light and a lower fraction of it will be red light. So the reflection would appear blue.

This is pretty much the same effect that makes the sky blue. Tiny particles in the sky don't reflect much of the sun's light going past them, but of the small amount they do reflect, more of it is blue than any other color.