Because the pigments in objects absorb or reflect light, and the reflected light which we recognize as a specific color. Each person or animal could recognize that color as something completely different than another. Also, because color is only a reflection of light, what we recognize may not be an inherent property at all.
I don't think doctors are supposed to do that but if they are I'm guessing it's probably to kill a infection of some kind or speed up the healing process
