Look to the military modelers to get metallic colors right. There are several different versions of “gunmetal” that could work for manhole covers. I haven’t gotten that far yet, but I’d probably just use dark grey as a base, with a wash of oily black and a drybrushing of burnt sienna.
Manhole covers are cast iron or steel. They are painted black at the factory. If I were going to have one on a rialroad I think I would paint it rail brown and then add very thin layers of black until it gave a blackish hue to it