Having just seen the "Mr. Spock, Mr. Spock" feature I can confirm that it's Nimoy's own hair in Into Darkness
, which actually looks much greyer than in the movie.
Nimoy's reaction when he learns who the baddie is is pretty cool, as is learning how the Spock/Spock exchange ends...
SPOCK PRIME: He will not hesitate to kill every single one of you.
SPOCK: Did you defeat him?
SPOCK PRIME: At great cost, yes.
SPOCK PRIME: I took a hammer...