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’m not sure if this is the right answer but the doctor would continue you to take it the few days after you have been feeling better to ensure the bacterial infection is completely gone?
