Zig Zagged: The suspicious looking character with white hair is actually on the good side, but he turns out to be The Mole. However, as soon as he reveals this, he dyes his hair. After which he goes back to the side of good because the hair contained a tiny mind-control device implanted there by the Big Bad, and it's rendered useless by the dye.
Averted: Who has white hair and who doesn't has nothing to do with whether they're good or bad.
Enforced: We need a way for audiences to tell apart a villain from his heroic counterpart — let's give him white hair so they can notice right away.
Lampshaded: "Fear me and my white hair of death!"
Invoked: A villain makes his hair white for the sole purpose of making himself look more evil.