No, not neccessarily. Harry Potter is an example. He has dark unruly hair and a lightning scar on his forehead. Skin colour? Does it matter? Did I think about it? No.You picture what they look like, yes? And I'm guessing you don't picture them in gray, white and black? Or do you just imagine their skin being transparent?
This is, for me, about how we suspend belief. How often are characters described in terms of their appearance? Usually, thats done in or around the point at which they first appear in the story. But after that? Usually, the focus moves to how they act and why, and how this drives the plot forwards. Appearance becomes less important and in some cases irrelevant. As a reader, I'm more interested in the characterisation and the plot, and I don't think about appearance.