How do I get a yellow card back?
Fireside Gathering.
You need to challenge a player on the same subnet as you 3 times while at least one additional player (3 total) is on the same subnet. There are sometimes real life gatherings where people meet to unlock this card back, but it also works with specific software or network emulation tricks.
http://us.battle.net/hearthstone/en/blog/13665271/pull-up-a-chair-introducing-fireside-gatherings-5-8-2014 Here is the link from the battle.net You need to clear 3 criteria
At least three people (including yourself) must be logged into Hearthstone on the same subnet.
Each match must be played against a player that is playing on the same subnet as you.
These matches can be played against your friends or using the Player Near Me feature, as long as the above two criteria are met.