Add me to the list... I have done 2 exchanges in total, meaning I went through 3 different iPhone 6 devices and all three were substandard: first had yellow tint, second had extreme shading in top half of display, and third had the infamous color temperature shift from top to bottom. I just returned for a refund after that.
I then decided to "downgrade" to a 5S, since I traded in my iPhone 5 (never an issue with the screen) and essentially had no phone. I tried 3 different iPhone 5S devices in the store and issues were much the same! Extreme yellow tint or yellow spots on screen.
I ended up getting a used iPhone 5S from a total stranger, and the display isn't perfect but better than the brand new ones I've tried. It is really quite baffling. I have no idea how I lucked out with my iPhone 5, but it seems that the current production standards are quite low.