Yeah, they like to hide this, but well know that they are required to provide warranty. One of my iPhones had discoloration around the screen edges and I bought it more than a year ago. They repaired it no questions asked.

I think in such cases the 'burden of proof' is fairly light. There is no easy way I could have caused the discoloring, except if the phone became wet (this was before iPhones were water resistant) and they could easily see this with the water indicator strip.