Madara looks better with black hair. I don't see why he got white hair when he became the Juubi Jinchuriki anyway. Atleast it made sense when obito did it because it looked like he was using Gedo Rinne Tensei.
Anyway, Hair is supposed to turn white when you use up a lot of chakra, and it seems that madara's chakra limit is equal to X, Where X is equal to X+1