There are black Gallyfreans?
We saw a black Timelord in "The End of Time, Part Two."
Which makes sense. Why wouldn't
they have Timelords with brown skin? Hell, the entire idea of race falls apart when the species can change bodies all the time.
Skin colour used to be about the last hundred thousand years our bloodlines spent outside in specific "unchanging" climates.
Gallifreyan's, have spent the last hundred million years indoors.
Of course different cities might have specifically different static climates, that if your bloodline decides to stay put for a hundred thousand years, then the colour of your descendant's skin is eventually going to be exactly what that city says that it is.