Gaiman has always been good about not making too much of a point about what skin color his characters have. It's not the color thats important but the character. Reading American Gods I never really thought much about what skin color Shadow had, but then when they casted an actor in a movie, I realized I thought he was not white (the actor cast seems not white too?). First I thought he was whiter when it turns out Shadow's mother is Norwegian, but then when Shadow goes to Iceland there is a line about how Shadow's mother didn't look like the women there, slender and pale.