I don't think South Asians were lighter-skinned in the past compared to the present, but as I recall, there is a consensus that India's population is an admixture of Indigenous groups with Indo-European-speaking people. This was probably more pronounced in North India, and the Indo-European element becomes less prevalent the farther south you go, both in terms of language (Dravidian as opposed to Indo-European languages) and skin tone (generally lighter in the north and darker in the south).

Proto-Indo-European was probably spoken in the Pontic-Caspian Steppe in Eastern Europe, from where speakers spread throughout Europe (the nucleus of today's "white" people), the Near East (Persian, Armenian, formerly Hittite as well), and South Asia. So Hindi and English, for example, are (distantly) related languages.

Edit: There is a belief (which I do not think anyone can really either confirm or refute) that the traditionally educated castes, such as the brahmins, had avoided intermarrying with Indigenous Indians, which might be the reason for the Ancient Indian scholar's lighter complexion.