I think your dog just does that because of the shedding season. During the summer the fur is light and thin, and during the winter it gets thicker, darker, (and softer too, right?) so the dog can stay warm during the winter. My cousin's Aussie/Pyr mix used to do this too, but he was a mostly white dog so you couldn't see much of a color difference. I know that Huskies (as well as some other long-haired dog breeds) have two types of fur - the longer, rougher top coat which you see during the summer, and the shorter, thicker undercoat of the winter that grows in during the winter season.