MIne isn't yellow but it has the same darkish mura spots as yours and I'm sending mine in for service/replacement. There is a real problem with quite a few of these iPhone 6 displays! I hope I get one with a good screen! I can't stand the dark shadow spots on mine! When reading from a white background it drives me insane! Never had this problem on my iPhone 5.
Heres another other thread with the same type of issue.