There is one, and only one solution for plastic bumpers when they fade, and that is paint. You can try all the expensive "bumper shiners" and "bumper blackeners" that you want, but they all fade after a few weeks. If you want your car to look good, paint them with a good quality bumper paint, or a body shop has some additive for paint that makes them work on flexible plastic surfaces. I'm not a panel beater so someone in the trade will know what I'm talking about better than I will.
And on the 7th day, he made Alfas....
There are only 10 types of people.... those who understand binary, and those who don't.