What determines hair colour?

Ginny - Well, the keratin that makes up the structure of your hair is actually white, so it's all about these two different types of melanin which are the pigments that gives your hair colour. There's eumelanin which can be black or brown and there's pheomelanin which is sort of a reddish, yellowish colour. It's the proportions of each of those that make your hair a different colour. So if you've got brown hair, you've got lots of eumelanin and not as much pheomelanin. Blondes will have any real proportion of the two, but not very much of either of them and red heads have a lot more of the pheomelanin and not so much of the eumelanin.