What kind of paint do you use?
It's a cross between acrylic paint and ink, so that it seeps down into the leather.
Does it rub off?
No, it won't rub off because it has been partly absorbed by the naked leather. However, it will scratch, as will the leather itself, so take special care with rings, keys, bracelets, etc. to avoid scratching.
What other kinds of materials do you use?
I use natural, naked, veg-tanned leather, YKK zippers, and solid brass hardware.