I painted my center caps when I painted my 97's OEM wheels a few months ago. Scuff them up well with either a red scotchbrite pad or sandpaper and then sand smooth; use a good primer (I used a plastics-specific one, but you can use whatever you want), and then on top of it use the closest paint match you can. I just used the same wheel paint I used on my rims and then used two coats of clear; AutoZone sells a brand of paint called VHT that has wheel specific spray paint in a silver color that would match pretty well; or, you can go crazy and choose whatever you want!