Pedro Pascal.
Link below (beware the MASSIVE shitstorm in the comments section though; much more vitriolic and racist than even the "Importance of Dorne part II" post garnered).
Hmm.. what's all the fuss about this time. People don't find his skin tone dark enough for the part? My Chilean/Argentinian friends are as pale as he is in winter. After a few days in the sun in summer, they get a killer sun tan.
He shouldn't have any problem pulling off a visually credible Martell Prince.
Of course as long as he won't be in character (costume, but also attitude and body language) he doesn't look that much like the Red Viper.