India is a caste society. The highest casts are largely descended from Aryan tribes that invaded and conquered India long ago. Consequently, high caste Indians are white, the lower castes tend to be dark, including very dark Dravidians.
Couple points.
This theory is almost universally held by historians and other scientists outside India. In India it is considered by many to be politically incorrect. Their notion is that the Aryans originated in India and spread north and west from there.
Some Indians get remarkably angry when their ideas on this point are challenged. Psychologically I think this is based on the, to me, very odd notion that the commonly accepted theory is trying to say Aryan Indian society is really European or "white" in origin, and therefore somehow to denigrate Indian society.
The "color" of Indians is more related to geography than caste. While I'm sure the "white" upper caste and dark lower cast idea has some relationship to reality in some areas, in much of southern India, for instance, the upper castes are just as dark as the lower ones.
The Indian word for caste, varna, actually means "color," BTW.