I believe just her skin would be dark. It's totally plausible for her to look like her father, while having her mother's complexion. It's the trend in all of the Sand Snakes. Tyene has her mother's fair complexion the same way Lady Nym has her's, but both resemble Oberyn. The reason I think Sarella looks more like Oberyn than the other Sand Snakes is because she's very androgynous. It is highly speculated that Alleras the Sphinx in Old Town is really Sarella Sand cross dressing so she can become a Maester. If she can trick an entire academy of learned men into believe she is a more effeminate man, it's easy to see how she would strongly resemble Oberyn!