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!
THAT LITTLE SMIRK; it's just such a reflection of his character that I've been wanting to see on screen for so long and it's perfect <<<true. People complain about how AOU's Quicksilver was too cocky and smug, and say that DOFP Quicksilver was better because he was funnier. They tend to forget that the Comic Quicksilver was a dbag.