The fading is some combination of the lower thermal conductivity of the bumper vs. the metal panels and the plasticizer used in the paint on the flexible panels. Sometimes those surfaces can be brought back, and sometimes they can't. On a newer car I'd bet you can bring it back (unless you live in the southwest where the UV is brutal), unless this is a repaint or something, which is sometimes hit or miss as to whether it's better or worse than factory. Just make sure to keep some LSP on that bumper if you're successful at restoring it.