Outwardly, the character of Harry Dresden is white but more dark than say Spike, but he does wear a duster and he has a hard boiled detective voice. I thought JM did a really good job and he will now forever be the one I see when thinking of Harry.
Harry's looks aren't described in great detail because as the hero he's supposed to be a canvas on which the reader can project himself, so I don't think Harry's hair-color is of any great importance to Jim Butcher. I think the vibe is more important and I think JM would be the perfect actor to play Harry. *Sigh*