It's not about supremacy, it would have been more about preservation. While my opinion is that the caste system was focused towards varying occupations among the different classes, there is an ethnic tinge in it. The dominant castes were generally purer Indo-Aryan. In any case, it would have aided in preserving the fair skin or the more white Caucasoid features. Everyone is aware as were those in the past that mating with a dark skinned woman would result in a dark skinned child & the appearance of the father would be absent among his descendants. The odd thing is that a man from an upper caste could marry a lower caste woman, and the child while probably not being close to white Caucasoid would still belong to his father's caste. As far as the popularity of fair skin is concerned, a considerable number of people are desperate for white Caucasoid features, & are thus desperate for partners with those features. That might originate from an ingrained sense of inferiority.
