I've seen black squirrels in the Lake Erie area.
We have some white ones around here.
I wish I could find a picture my ex took of a squirrel he found at work one day. It was bi-colored...whitish with a pattern of reddish brown, like a dog. It was dead as hell, but I have not before or since seen a squirrel like that one.
edit: after I posted this I googled squirrel colors and the first hit was this from the Cellar:
http://cellar.org/showthread.php?t=12506