My vest started out as steel blue, then I misted on charcoal grey. If your vest is made of vinyl, you can use SEM coatings. It's made for flexible stuff. I get mine at an automotive paint store.
I second what Jango 5204 said. I think the main color of the leather is a slate blue, but there was a heavy weathering of grey on top of it. If you look at some of the reference pics in the gallery you can see the underlying slate blue color.