The "main" ethnic groups in subcontinent tend to average about 10 - 30% of the "group" having strong admixture of other ethnic groups hehe.
We all invaded/got invaded migrated to/from for many millenia hehe.
In middle of darkest skin Tamil Nadu for example, there are very north indian looking people because they came from Gujarat+MH originally and settled down and adopted the local customs and language (and also gave some counter-influence on locals) and also inter married etc. They stem from the Maratha kingdom cheiftains coming and setting kingdoms in TN (in void created by collapse of Vijaynagar)...and also Gujju traders
My own family for example have more north indian looking ppl and more traditional south indian look too....because of way we married within our caste (Iyer Brahmin) but there being variance because of the migration norms of our particular gotras over time (and there has to be some intercaste marriage/caste graduation across much time too given the much different skin colours among say my grandparents).
This is just one example, there is much regional variation too....but one needs to be from the region to spot well.