Like Ged and Ogion, Nemmerle is from Gont, so he probably has the same brown skin. But he's so old that "His hair and beard and robe were white" (3.15) – because your clothes turn white when you get old, it's a scientific fact. He's also a very nice man. He's so nice that he sacrifices all of his magical power in order to get the shadow monster off of Roke, which ends up killing him.