You don't want to paint that. The purpose is to guard your bumper from scratches when loading something into the trunk. If you paint it the paint will scratch off and it will look horrible. If you don't like it just remove it. If the plastic is fading and you don't like that there are products to fix that mothers back to black is one.
I had one when I first got the car. I tried sanding and painting with bumper paint made for plastic, within a few months it started chipping. I ended up taking it off. It looks a lot cleaner now IMO.
You can remove it and then get some Black Krylon Plastic Paint from Lowe's to bring it back to life. This paint is meant for outdoor use ( ie: Plastic Patio Chairs, Mailboxes, kid's toys, etc....) Then when you put it back, use 3M tape around the whole edge to keep it from popping up in the sun.
I did this to mine, and you would never know that it was painted. Now it will remain looking like new.