If you check an Arts & Crafts Store like Michaels, they sometimes have small boxes of pastels that are called Earth Tones. They will contain black, grays, browns and sometimes greens. These are the best to get for armour modelling IMO. Be sure that you pick up pastel chalks though and not oil based pastels (they're just glorified crayons).
For your particular model, I'd probably go with some browns worked into the nooks and crannies with an overall dusting of gray.
Happy Modelling and God Bless
Robert