Over the past 5 years I have spent a lot of time trying to mix up the "perfect" natural paint. But all paint binders have their pluses and minuses. And a binder that is great for watercolor paintings may not work well for mask-making, where flexibility and moisture-resistance are of great importance.
Another factor to consider is how easy the paint is to work with. For example, an oil based-paint that is slow to dry and difficult to clean up may not be ideal for a children's mask-making class.
This document shares some of the paint binders I have been using. There is no one "perfect" binder here, but I hope the information will help you narrow things down if you are ready to paint your mask (or other art project) with natural paints but have been wondering what to use.