I have used the same color that was used for the body on the 2,7 and 9 series cars. That paint code is 130.

I will point out that 30 years down the road the original finish on the center caps will probably not match that Silver Metallic 130. I found that the original finish on the 30 year old center caps was a bit darker than the new 130.

I thought about repainting the caps as well as the wheels and having decals made to duplicate the original graphics. That was more than I wanted to spend. I decided that if I moved forward with repainting a set of Hydra wheels I would just purchase new caps and sell the old ones on eBay to off set the cost somewhat.
--
Any twenty minute job is just a broken bolt away from a three day ordeal