My guess is that coz he's super fast, parts of his body are aging quicker which is why his hairs going grey. but its silver because if you cans see in the second photo, his brown hair has a silvery sheen which is coz of the powers and that's why his grey hair is actually silver!
Why so sad ? Heath Ledger as the Joker skate boarding over Christian Bale as Batman while they take a break on the set of The Dark Knight. You can all quit your lives now. Single greatest picture in the history of pictures and internet.