The bumpers were repainted by a crappy body shop at some point before you bought it. Not much the warranty can do about that i'm afraid. However, The dealer you bought it from might be willing to pay for them to be redone. For that matter, they MIGHT have been the ones who had the bumpers repainted in the first place.
At the dealership I work for we get used cars in all the time with scraped up bumpers. There's no way a car will sell like that, so we have to have them replaced or repaired and repainted.