Hello!
Steel has so many different colours - depending on the individual alloy (stainless?), how long it has been sitting outside or if it has been rubbed on (think manhole covers, for example).
My take is to use some kind of gun metal paint (the so called "leadbelcher" from Citadel paints has a very nice colour) and adding some brown to it or using a brown wash on top of that. You can also mar the very edges with silver to suggest most recent wear.
The graphite stuff can probably be used, too, I'm just not so sure about places that could be handled - it would probably tend to stay on the fingers a little...
Good luck with your builds and have a nice day
Paweł