Milk paint has been used for thousands of years. It is extremely durable, often lasting for hundreds of years if protected from the elements. It is made of all-natural ingredients, and is very safe and completely non-toxic. Because oil base and acrylic paints are made in vast quantities using cheap petrochemicals, their price can be low relative to the price of milk paint, which is sold in smaller quantities. Milk paint is commonly sold in the powder form, which is a combination of casein and lime. Once water is added, the lime activates the casein and yields a durable but caustic paint, that can only be used on porous surfaces. Milk paint comes in a variety of natural pigments and colours and is very safe for the environment.
Milk, lime, vinegar.
How to make your own milk paint (Article)