I've always thought that colorism was a result of colonization of white supremacist ideology. Black people have felt pressured to conform to white ideals of acceptability, so denied their natural hair and tried to whiten their skin. W.E.B. Du Bois noted these behaviors 120 years ago.

Interestingly, there is also a form of colorism among white people. It goes back centuries to the idea that having a tan meant you worked with your hand outdoors and thus were lower class. This prejudice was especially applied to women, and the mystique of the lady fair of face made her seem classier.