No, it has nothing to do with the tinting whatsoever, or lighting. Bears change color between various tilesets, as well as Hawks (Beastmaster summons). In Ashenvale and Felwood tilesets, they automatically appear purple. In Icecrown and Northrend tilesets, they appear white. In everything else, brown. There's not different skins, listed at list. Polar Bears are different, because they're thier own skin/creep. I'm referring to the summoned Bears/Hawks.