Do you have an example of this? I can't remember them ever having production issues that caused the phone to deteriorate.... Maybe the iPhone 5 slate black got a bit discolored after a bit, but that was metal. The color is under the glass. Unless you smash the back glass the color can't go anywhere.
The white iPhone 4/4S's sometimes would yellow over time. Just because it's behind glass doesn't mean sun exposure wouldn't affect the paint.