Look up http://www.duplicolor.com they make a whole line of factory matching paints, plastic primers, and plastic "adhesion promoters" They should have what you need and in a matching color. Depending on how big the damage is, you can opt for the brush-on or spray on versions of the product.
Most bodymen will attest that silver is the hardest color to match, though.
Go to http://www.paintscratch.com/ they have Silverstone Metallic and it's dead on. This is a duel stage system so it's easier to color match. Dual stage means you apply a base flat coat of paint then spray a clear on top.
I'd get spray cans for both the silver and clear paints. I think they can formulate it for plastic bumper adheasion as well.
I've used their paint on my Insight and you can't tell where the touch-up was done. Good stuff.