Well, if you are talking about physics, it is the color it is because it reflects certain wave lengths of light, giving it a certain color. If you are talking about biology, it is genetics. You get one gene from each of your parents and, based upon the genes they give you, a hair color is expressed. Darker hair is dominant.
if you have dark hair you could have two dominant genes (XX) or one dominant and one recessive (Xx)
if you have blond hair, you have two recessive genes (xx)