Several bosses have different color circles around them. On one loading screen it said that the orange causes fire damage to you, and I've fought a few things with a blue-ish one that drained my focus, but I've seen others that I can't figure out. What do they all do?
There are five auras:
Note these colors roughly apply to your characters' abilities too. For example, the badges for Anjali's fire talents and abilities are orange-tinted, and many of the healing abilities are green-tinted. I guess you can consider it to be Dungeon Siege III's version of an elemental system.