Cook some paint! Or not, as this really doesn’t have to be cooked. Cooking changes the consistency of the starch used, so do some chemistry experiments, or just throw the ingredients into a jar and shake.
Flour (Rye or Wheat)
Red Iron Oxide
I left off proportions, because everyone wants a different consistency with paint. Rye is the traditional choice for flour, but wheat is less expensive. Cook those with the water, then add the pigment–there are tons of natural pigments to choose from, and a very little goes a long ways. My advice is to stick with the mineral ones, as they are fade proof. I use food grade linseed oil for interior paint, and nasty boiled linseed oil for exterior paint. It just dries faster.
Once you get into the natural paint deal, you may never buy prepared paint again. To steal a comparison from Wendell Berry, it’s like the difference between real food and industrial food, industrial food being like industrial sex.