I don't think the touch up route will work. You'll need to remove and respray.
I might go one of two ways. Tower Paint (if they are still in business) will give you a VIN# color matched can of spray paint for about $10. You'll have to buy a can of clear coat. Or maybe you can use the Wurth paint kit which is the same silver as your wheels. If the parts are plastic you may need a plastic adhesion promoter which you might be able to find at Autozone or a body shop supply store.
Before you do any of this you will need to thoroughly clean the surfaces using alcohol or paint thinner.
There's also a guy on Ebay does this type of stuff. I think his user name is asimplechef or something. Maybe you can remove the parts and send them to him for painting. He only does silver for some strange reason.
here's a link to the Wurth Kit being used to repairs wheels scuffs.
http://www.bmwtips.com/tipsntricks/wheel-repair/wheel-scuff-repair.htm