Is the spelling for gray hair gray or grey?

That depends on what part of the world you are from. The English spell it 'grey' and the Americans spell it 'gray'. in Ireland we spell it grey xx

gray is american, grey is the "english" (or british, whatever you want to call it)

2 ways oldness and stress :)

the pigment is fading in your hair

definitely...grey in British English, although the preferred American spelling is gray