I originally found this recipe in Do It Gorgeously by Sophie Uliano, although there are lots of different recipes in various places online. I loved the book as a whole and highly recommend it. You should also check out Sophie's website, it is a great resource.
1 Gallon of Skim Milk at room temperature
2 c. White Vinegar
200 g. powdered color pigment
112 g hydrated lime powder
2 glass or plastic containers with 4 cup capacity
1 Large Colander
1/2 yard cheesecloth
2 1-gallon paint buckets
2 large wooden spoons
Stir vinegar into the milk. Allow to stand overnight. Milk solids will begin to separate. In a separate container, mix your color pigment with an equal amount of water and form a paste. In a third container, slowly mix together lime powder and 1 1/2 c. of water until is has formed a creamy paste. Using your cheesecloth, separate the milk curds from the whey. (The curds are the solid part, they whey is the whitish liquid that remains.) Put your milk curds and lime paste into a paint bucket and stir well. If there are lumps after stirring, wait fifteen minutes and stir again until it is smooth. Finally, add your color pigment! Stir well. The paint should now be about the consistency of light cream. If it is too thick, add a little water. Strain the paint through the cheesecloth one last time into your second paint bucket and you are ready to go!
Using a finish over your paint job will protect your work. Find finishes like Zero Gloss or Black Bison Paste Wax, along with pigments at Earth Pigments Company.