When they're colored, it means they've been "tributed" three times, which is the max. It'll do that with your cards too if you tribute to a single one three times and all three succeed (you should also see 3 little icons at the top of the card if it succeeded).
Ah-ha, good to know. Especially since wow, the Champion is an outrageous difficulty spike compared to the Elite Four. Time for a whole lot of tributing...
