Gualdo, I agree that it is very interesting that Jon has black hair while Eddard has brown. I don't have time to pore over the books for references to hair color right now, but I wonder if the hair color is a game that Martin has been playing with the readers. I mean perhaps both characters have only been described as having "dark" hair in the books so far but later in the series the fact that Jon has black hairEddard brown might matter. This kind of detail could be hidden over several books but there's nowhere to hide in a visual medium. I guess we won't really know if this is the case until the rest of the books come out.

This is mere speculation, but I'm thinking it's most likely that Sean Bean didn't want to dye his hair

Maybe I'm must biased in that althought I generally Sean Bean as an actor, I'm not sure I really like him as Eddard. The books make it pretty clear that of the children, only Jon SnowArya share Ned's hair coloringfeatures. This is an important reason that Cat dislikes Jon so much. Maybe it's just the previews I've seenfocused on, but Bean's hair looks very light brown to me, certainly not dark,not at all like JonArya's. They can probably pass over this easily enough in the series. But, to me, the hair color, though not obvious, is really a major plot point for all of the houses throughout the books, from the Baratheons, to the Lannisters, Tully's, family dynamics within House Stark, even down to the minor houses (actually a lot of red hair color in the books, but I digress...).