Discoloration on back of black iPhone 5

Hey guys,

Swapped my black iPhone 5 32GB a couple of weeks ago at the genius bar as it had dust trapped under the camera lens. After a couple of days using the replacement one however, I noticed some discolouration to the back of the phone (almost like a darker band down the middle or two stripes either side of the apple logo depending on the angle you look at it from - please see attached photos). Has anyone else encountered anything like this before? I noticed there were a few discussions for discolouration around the antenna lines but this is far more annoying to a perfectionist like myself! I tried cleaning it with a glasses cloth incase it was an oily build up but it remains exactly the same. Is it possible this is a problem caused by the battery overheating? Also would anyone recommend going back to the genius bar? I know it seems something very trivial, but for a £599 phone you would expect it to be fairly perfect, especially with Apples standards.

Thanks,

Stephen