It's not really a continuity error. I mean, you can dye your hair, and while a Vulcan wouldn't usually do so, perhaps Spock felt the need to do so for some reason of which we're not aware. To me, a continuity error would be something like having pointed ears in the first movie, and round ones in the second. Something as simple as hair color over the course of 4 years between two movies doesn't really strike me as an error.
If that were the case, Scotty would be one massive continuity error from Star Trek's one through six.