For the uniform colour, try this link: http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=489
This will take you to a discussion on Field grey in the Figures section.
For faces there is no quick answer, I use Windsor and Newton Artists oils, over a base of desert yellow/flesh mixture. The techniques with oils involves laying down the colours, then brushing away almost all the paint, esentially leaving just a stain. Starting with the darke shadow colours, then working into the highlights.
For brown leather, start with a medium brown paint, then using oils apply a thick coat of darker brown oil paint, and using a completely dry soft brush, carefully wipe most of the oil paint off. this will leave a nice finish, but you must leave it for about a week to dry thoroughly.
For black leather simply paint with matt black, then dry brush with burnt sienna oil paint. this will impart a nice subtle sheen, and add a hint of brown, just taking the edge off the black.