In the pic I posted old Spock's hair looks darker, but overall he looks younger anyway. Nimoy may have dyed his hair, or it may just be how his natural color looks when cut and sprayed down.
The main reason I think it's his real hair is the flat spot just over the center of his forehead. If you look at some of the TOS episodes, his hair has the reflection that follows the curve of the bangs, but about half an inch above. In several shots there is the flat spot that does not follow that curve. That's what I saw in the screenshot from the new movie.
Also, look at the cut of the hair at the temples and around the ear. It looks to me like natural hair, and not the type of wig that he wore in the 09 film. When that movie was shot, he was wearing his hair much shorter, almost a crew cut. In candid photos from 2012-13 his hair is much longer. Look at the Audi ad. It's not at all unreasonable for me to think that he let his hair grow out so he wouldn't have to wear the wig.
I could be wrong, but that's what it looks like to me.