Given how Spock Prime's ears changed shape between each and every film from 1979 until 1991 and then looked different again in "Unification(TNG)" I'd say his hair color in the new film is a pretty minor nit.
Spock dyed his hair or something. He is, after all, 158 or 159 years old by this point, and half-human to boot. It's a wonder he doesn't look a lot older and worse.
btw NuSpock's hair is quite.. plasticy looking at times. Is it all real?