see if you have a local "quick paint" type place. When I worked for a dealer these fellas would come in and fix little scrapes all the time. Did it in the parking lot too.
If I were to do it, I'd nab a can of paint, a few scotchbrites and a couple cans of this awesome freakin' clear coat:
That clear is about 20 bucks a can, but holy SHIIIIIIT its awesome.
Lightly scuff the area, clean with alcohol prepped wipes, and go to town. You will need to "blend" the paint, which would be tough as **** with a spray bomb (you can't thin it down). But, you may be able to feather it? Who knows.......never tried that. Then clear the entire bumper. Wetsand and buff. Yum.