IMO, that gunmetal grey or even as dark as charcoal looks really really good, EXCEPT on silver cars... stick with silver.
edit: he's probably just worried he won't be able to match the same shade of silver again. Wheel Wizzards (not recommended!) in Byfleet tried that with me a few years ago and one of the guys mentioned that somebody had complained when the guy came in with a custom wheel colour and they didn't match it... I just said that as far as I was concerned, Avus silver was Avus silver, so as long as they used the same paint they did for Audi (who they claim to refurb wheels for), then I would be fine with it... just a same that they then didn't cure the wheels properly and stacked them in cardboard boxes (leaving corrugations all over the rims) and then took two weeks to sort out the wheels and still did a pretty average job of it.