CaptT,
First of all, welcome to the forum. I hope you find your time here "well spent" and productive...
You posted your comments in the RV Service & Warranty Issues section of the forum. I do hope you aren't expecting Keystone to provide warranty service on a 6 year old trailer with a cosmetic problem. You'd probably be much further ahead talking to your dealer about that kind of action and getting the service manager's advice on how to proceed.
As for fading, typically almost all the plastic accessories (vents, shower doors, refrigerator vents, water access panels, etc) yellow with age and exposure to UV rays. Look at almost any older RV (more than 3 or 4 years old) and you will notice yellowing of most trim pieces. Even trailers with gel-coat FILON panels will "weather differently" based on where the panel is located. Vacuum formed FILON walls with 2" of foam bonded to them will weather differently (and fade differently) than panels with no backing, like panels on access doors. Typically, the first place you'll notice the different shading (from fading) is on the "people door" which is usually faced with fiberglass sheeting rather than FILON sheeting. Often, you'll find that access doors are built in one plant (often by a sub-contractor, not by Keystone) and trailer sides/walls are built in another plant. Usually the colors match very closely when the coach is new, but as the years accumulate, fading due to UV exposure will affect the color of different brands of FILON and even different lots of FILON at differing rates. So the FILON on one side of your coach may not match the other side, although with the separation, you can't compare them "side by side" so you'd never see the fading. On your access doors, chances are that they are constructed differently and will fade/yellow differently.
What can you do? I'd suspect that you'll be in for a long, frustrating journey if you try to demand that Keystone repair your trailer under warranty. First, the warranty expired years ago, secondly, and I think more importantly, cosmetic issues are not considered a "defect" and are specifically excluded from the warranty. It's in the owner's manual, in the warranty exclusions. If you don't have a copy, you can download the correct year in the Owner's Manual Archive at this link:
https://www.keystonerv.com/owners/owners-manuals/
Again, welcome to the forum, but I'd opine that you're about to start an uphill battle that will be nearly impossible to win. Good Luck