The different background colors on Experts indicate how rare they are. The more rare an Expert, the more Categories they have knowledge in, making them more helpful when asked upon.
  • Rainbow: Epic (6 Categories)
  • Gold: Rare (5 Categories)
  • Purple: Uncommon (3 or 4 Categories)
  • Gray: Common (3 Categories)
Rarer Experts also require more Cards and Coins to upgrade compared to the more common ones.