You want GM Part number 12346693 (Code 67 WA994). I actually bought mine at the local Buick dealer for around $10 (it's $5,72 on gmpartsdirect.com). It's the kind with a ball point and a brush. It matched perfect. See the photos of the rear parking sensors in my gallery - they were painted with the touch-up.

 

I purchased mine at the dealer for just under $10 w/tax.

 

Compound it first, maybe hit it with some 2000 grit sand paper, then compound it and polish it. Most scratches will buff out. If that doesn't work use clear first that will cover most scratches with out having to worry about matching. Metelalic flop on touch up is very hard to duplicate. If the dealership has a body shop they will probally give you some to touch it up from the body shop.