Casein paint, or "milk paint" is very different to work with. It is an opaque watercolor that actually dries flat, and eventually gets hard. The colors are more grayed, which I happen to like. There are many articles on the Internet, if you Google "casein painting". There are even a few tutorials.
This is the piece I started last week. I'm not sure that it is finished. I may need to do a little work on the faces, although I do not want them detailed. It is good to put the painting on an easel for awhile and see how you feel after a couple of weeks, or even longer. I like the subdued colors for this subject matter. Anxious to start another one soon.