For the recipe, hop over to The Imagination Tree's post.
First mix the cornflour and cold water together, then add a cup of boiling water at a time and stir between each one, then keep stirring until you get a wonderful white goo, which is itself a great messy play material to play with. You can then divide the mixture into as many different bowls as you like and add a different colour to each one.
Because of the white base mixture the paint has a nice milky/pastel-y tone to it which is different from most other paints, and it has a more sculptural feel on the paper too so its a great sensory alternative from ordinary painting! Completely washable too!