Two ways to go, that I can think of quickly:
1) Paint the caps...
2) Measure the diameter and shop around. Most of these things are pretty standard size (IIRC, 74mm?) even if the wheel center bore is, I think, 63.6mm. Anyway, measure to be sure, then surf the web for "center caps" or check out your local wheel and tire stores for plain ones.
3) (OK, three, I lied) I wonder if the backing would come off? People have done this kind of thing with their Ford badges. Maybe check out the aesthetic forum for ideas.
