I'm not so sure that painting is a good idea. Paint leaves an inflexible layer that will crack and chip off at the slightest wrinkle of your mask. I feel that there are a couple of ways to go about this:
1) Dye - probably need to prepare the surface of the rubber to increase the dye's effect
2) Coating - something like liquid latex that can be cured? Or perhaps cut the mask along the seams, glue panels of rubber on the flat mask form, and then reglue with internal reinforcement?
Dye would be far and away the cheapest way to go, but I am unsure how effective it will be. Still, I would be interested in your experiment though!