Srilankan Tamils are purer Veddoid or Munda(base race is Sentinelese Negrito). They are the ancestors of all Dravidian speaking peoples of south-India, Malaysia & SriLanka.
Indian Tamils, due to following Hinduism & Aryanization of their culture, got mixed with Caucasian (Eur-Asian) genes , at the same time. The same way non-Caucasian aboriginal races of the subcontinent received Caucasian genes from Caucasians who also brought them Aryan aka vedic-culture
That’s why a few Indian-Tamils or south-Indians look a bit Caucasian, though their mixed features are quiet prominent (without makeups, hair straightening & Plastic surgery etc.)
The amount of mixture among Indian Tamils is so much that the non-Caucasian features get expressed in offspring. Sometimes offspring look different than parents. Arab or Caucasoid looking parents give birth to dark, Austro-Asiatic features
This is called independent genetic assortment
See how it works :
For Srilankan Tamils, they are very homogeneous & identical. (except very few Christian Tamils who mixed with European missioners)