I don't know if anyone would say that there's any rules to these arctic ponies or not. It's mostly just some fluffy tummies and hooves, and more natural fur and mane colors. All the manes I've seen have looked more shaggy and less curly than some of the show ponies, that's how Mac ended up with his mane over his eyes and Fluttershy's mane lost some of it's twist.
The bright, show color version is right over here: