How many gallons of methylceluose will 1 pound of powder mix up into, and are there other recipes of things you can add to it to get different textures and thicknesses? And if you mix in any dye, what kinds won't stain the skin?

1/4 cup of powder makes a warm gallon. It thickens as it cools, so you could stretch it even further if you want to use it cold.

I use a few squirts of Tempra brand kid's finger paint. The blue color will stain right out of the bottle, but once diluted, it's pretty safe even on bleached hair.