iPhone 6 screen yellow edges

Since half a year now my iPhone 6 16GB has yellow edges around the whole screen. I bought it when it was released in Sep 2014. The whole screen seems to be a bit yellow as well in comparison to other iPhones, but you will especially notice a yellow frame around the screen. The screen, as the phone itself, is not damaged neither was any part replaced.

If the screen is locked you can see the yellow frame as well. If you look at it under blacklight you may see it even better.

I tried to fix the problem with the "screen-adjustment" setting, but it didn't help.

Any advice or similar problem?

Thanks in advance!