By any chance, did you finish the Kalos Dex? Usually the National Dex is one of the achievements that changes the trainer card, but with so many pokemon now in it, maybe Kalos Dex is one, too?My trainer card was blue a few days ago. Now it's silver. Someone on the forums said the card was originally green. So it turns blue after you defeat the Elite 4? What's silver?
Alternatively, how far have you gone in the Battle Maison? The Battle Chateau? Have you gotten all the berry mutations? Perhaps one of these affects the card.