Colours

Derek's hair is black. Everyone else's hair changes colour.

Your hair shows your emotions, when they happen and as intense as they happen. Each emotion is a different colour. There's no way to hide it.

Derek dyes his hair black, feeling too vulnerable after his family dies to allow people to see his emotions. Everyone just assumes he is still grieving. This lets him avoid the rest of his emotions, as if they don't exist.

When you meet your soulmate your hair goes white until you seal the bond. Then it returns to its colours, but is streaked with the white.

Stiles' hair goes white as snow when Derek lays his eyes on him for the first time. His remains black. He knows it's from the dye. Stiles doesn't.

When Stiles still manages to work himself into his life Derek can't quite understand how he feels and all of a sudden his black hair is a problem.

Stiles wants to help.

Derek let's him.