Well in the first one "her hair" says her mom
Nancy freaked out and went insane, its normal believe it or not its somethign that has to do with cells (sorry but i really am not a scxientist) and like people can get gray or white hair from insanity.
*recalls sweeney todd ROFL*
But yah and in the third oen its just the same hair she had from the first it never changed

nancy went crazy

Ya know when me and my friend watched the movie, we wondered that ourselves

Im shocked to see someone ask a question about it too
My theory is that maybe in her sleep she had been around fire. Her hair could have had contact with the flames and I dunno.... went gray XDDD

Supposedly because of stress, traumatic experiences or extreme terror. Given the fact that she had just experienced extreme terror it turned here hair white. Same thing happened in the movie Poltergeist.