Racism

Skin colour is the result of evolution. Melanin is a protection against ultra-violet light. It's likely that, since the first humans originated in Africa, they had dark skin and that those that migrated northwards gradually lost the melanin because they needed all the sunlight they could get to make Vitamin D. Dislike or mistrust of people of other races than our own is really more due to culture than to skin colour, though the colour of a person's skin is one very obvious sign that they are from a different culture.