Casein paint has been used since ancient times as a form of tempera paint, the Egyptians used it and today it comes in tubes. One of the qualities that I love is that, unlike gouache or tempera -which I also love- is that it is not as picky and allows me to work better on transparencies and, while humid/wet, it behaves a bit like oil paint. When it dries it is resistant and has that oil-like visual quality, more so than most other water-based paints I've tried.
Milk paint is a non-toxic milk protein based paint to which I add color pigments. Milk paint has also been used for thousands of years and is made from all-natural ingredients that are safe and non-toxic once applied.
I painted with oil for years, unfortunately we are no longer compatible, the smell and toxicity of the products to dilute and clean are beyond me. I know there are water-based oils out there, but my head just can't turn around the concept and to tell you the truth, I'm a bit stubborn to try.
A friend made me discover casein paint, in fact she reminded me of what a great master Dauguet taught me, to make my own paints. He introduced me to casein, marble dust! oil tempera and exploring with materials...good good times learning with him.