An excellent diagram from Discover Magazine is pasted below:
Image-11
Some interesting quotes:
“The finding could mean that changing hair or eye color could be possible without using dyes. It would simply be a matter of taking a drug which changed the expression of certain genes to alter appearance, turning blondes into brunettes or preventing grey hair.”
“Preventing grey hair is a possibility and even reversing grey hair might not be impossible. Once we know more about the pigmentation process, and all the genes involved it should be easy to find a protein or enzyme to up-regulate or down-regulate the activity.”