We can be pretty sure that the Summer Islanders (Jalabhar Xho, the lead archer of the Golden Company, and the crew of the boat that took Sam & Gilly from Braavos to Oldtown, as examples) are typical sub-Saharan Africans. Alleras is the offspring of a Summer Islander and a Dornishman, and the description makes it clear that they are very distinct phenotypes, and his/her appearance is the combination of different looks. No one in Westeros would call a First Men/Andal crossbreed "exotic". And given that Alleras is likely to be the disguised daughter of Oberyn himself, it's not even like you can say her father was one of the blonde stony Dornish. The point is, Oberyn Martell is probably a white guy, for all intents and purposes.