I'm having a similar problem with card colors. I bought the base set and loved it! I quickly turned around and bought the other 5 adventure packs. Sadly, when I started pack 2 this evening, I noticed that the backs of the cards in pack 2 were a different color from the base set cards. I checked my other packs and found that packs 2,3 and 6 all have card back colors different from the base set. I've already order the character pack, but it hasn't arrived yet. I've got my fingers crossed that it will match the base set. I've just sent an email (with pictures) to customer service asking for help. The change in the color really affects game play. It's impossible not to know when a card from pack 2 is next on the location deck.
I really love this game and I hope that customer service can help me. This thread kinda scares me, but I'm hoping that since my issue is more than just the character addon pack that paizo will help me out.
Fingers crossed. :)