There is an old Hindi/Urdu saying which says all battles are fought only for three reasons - Zar (wealth), Zoru (woman) and Zamin (Land). The landmass we know as India now has traditionally been invaded for thousands of years by various races including greeks, kushans, parthians, caucasians, mongols, Arabs, Iranians, celts and what not. These races were not inhenretly Indian but originated elsewhere.
Every successful invador/conquerer plundered the wealth of India and either settled down here itself or left his genetic seeds in India. Calling it 'Rape' is not right, since the word in its present legal context did not exist till the advent of common law system introduced by the british. Prior to that, the victor/conqueror always exercised his right of first call on wealth and woman of the conquered territory and the consent be damned.
This mixing of bloods of various races has also been supplemented by traders , nomads and seamen from every other possible human settlement from rest of thew world.
This has resulted in today's Indian population becoming a a mix of various ethnic races including extremely dark skinned Adi-Dravidas in south, Australoids and Negritos of Andamans and others (may be dravidians) in various tribal settlements in central india, chota nagpur, bastar, jharkhand etc., extrmely fair complexioned people are found in Mangalore, parts of Kerela (offsprings of amorous seamen?) and Kashmir, Panjab area (invadors settled in Indo Gangetic plains and Doab zone), Mongols in North East etc.
If we honestly admit to ourselves, none of us Indians are of pure blood origins. We have mix blood from several races. So talks of brahmins being fair, fair being superior to dark, NBs being inferior since they are dark etc. is extremely silly stereotyping and has no scientific corroboration.