What determines skin tone?

Before anyone says the obvious, I know people that live in hotter climates generally have darker skin that those that live in colder climates with less sunlight. However, I was wondering what determines the actual tone of someones skin. For example, in Africa the skin colour is a different tone to that in India. Both are hot climates with lots of sun.

I am sure there is a very obvious answer to this but I hve been trying to come up with a logical answer and not having much luck!