They showed exactly what you're looking for on a couple of episodes of Making Monsters. Here's a recipe I found on the net:
Permanet blood paint for latex masks
Gloss Acrylic Medium (sometimes known as Gloss Sealer)
Bright red powder cosmetic pigment
Dissolve the powder into a little water first.
1.5 to 2 teaspoons dye to pint of Gloss Acrylic