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The old knowledge was Toyota Silver touch up paint. I've done that a few times and had great luck.
Clean the hell out of the center cap and go over with a Scotchbrite, hit with a spray of adhesion promoter, then 2 or 3 light coats of the silver touch up paint. Done.
I sprayed Rust-Oleum "Painter's Touch Metallic Aluminum" into a plastic bag and huffed it. Just kidding. I dabbed it out with a small brush and brushed it on the center caps like touch-up paint. The color is pretty much right, but it's slightly more reflective than the existing paint. If you see it at the right angle in the light at a close distance (3, 4, 5 feet), and are looking for it, you can see it. Anything over a few feet away you can't really tell. Not bad. I have another shade of silver spray paint on the shelf, I may try it.