Looks like iPhone 6 / 6 Plus cases my suffer discoloration plastic antenna band areas. This looks to be caused by friction if you have put the phone in your pocket. So might be an idea to buy a case for it!
http://www.neowin.net/news/iphone-6-...-discoloration
http://bgr.com/2014/10/22/iphone-6-c...color-rubbing/
http://www.neowin.net/news/iphone-6-...-discoloration
http://bgr.com/2014/10/22/iphone-6-c...color-rubbing/