She already has some gray hair because she has vitiligo (http://en.wikipedia.org/wiki/Vitiligo); there's a patch of it in her scalp. No pigment in the skin, no pigment in the hair. It's actually white hair as opposed to gray, and the texture isn't like most people's gray hair - it's very fine and smooth.

I am so glad so many people are watching Miranda! That way when the new series starts I'll have a bunch of people to squee with. :D