Sadly, if you are down to the bumper material, not anything easy that I know.
Well ...
A body shop could re-spray and blend the bumper - probably around $200 at a shop like Maaco (some of them are decent) $450-$700 at a more reputable shop.
The dealer probably has a $5-10 bottle of brush-on touch-up paint. It won't look as bad, but it won't look factory either.
https://www.google.com/search?q=touch-up paint&gws_rd=ssl#q=touch-up+paint+forum
If you look on the detailing forums, they recommend building the area up, then applying several coats of paint to above the original surface, then wet-sanding and polishing down. That should work well, but I've never had the patience for it or much success.