The easiest is just mix artist's acrylic paint with the liquid latex you used making the mask. The paint is flexible to a degree and when mixed with the rubber, it bonds well. Use rubbing alcohol to clean the surface of the mask first though, it opens up the rubber surface allowing the paint to penetrate some.
I didn't think you could actually mix acrylic paint with latex and that work?
I was thinking of just buying the monster makers latex mask paints in the colors I need, and then just mix colors to get the intermediate colors. They 7 bucks a bottle, unless the latex + acrylic or oil method works that seems to be about the cheapest and easiest way to go.. thoughts?
Stipple with the foam, rather than brushing across to get it more even. If you need it perfectly even, consider airbrushing it. But two coats stippled will usually get you there, with a but of natural variation.
I used this mix from FX warehouse when painting my Salacious Crumb puppet with much success! I want to say I mixed about 4-5 parts base to one part acrylic paint. You need to be careful though, the paint will dry much darker than it looks in your bottle, the base mixes in white but dries transparent/opaque. When you use a latex base to paint latex, it adheres VERY well, with no cracking. I followed up with a very light coat of latex sealer just in case (this one).
An artist's grade latex paint is more expensive than a student grade - it has more pigment in it, which of course makes it more opaque. I often use a sponge to stipple rather than paint, just as ChickenHaunt said. The latex rubber mixture dries and will hide most imperfections from the brush or sponge. I always thought this was the best solution, as opposed to rubber cement, as it is the same material as the mask.
The best method I have found is to airbrush your base layer with an acrylic and Pros-aide mix. You can mix in as little as 10% pros-aide and as much as 50%. Make sure to thin it out with water and alcohol to run through the airbrush. And you must thoroughly clean your airbrush after this step. After your base layer is down you can just paint with straight acrylic over top. Just make sure to seal your mask with Liquitex Gloss, Matte, or Satin depending on the finish you are looking for.