Drywall topping, especially the lightweight variety works well if you
put it on first then paint on top of it. It is cheap and fluffy. You
might try adding dry tempera to it, but I tried adding paint to it
and plastering my studio wall and it stunk like sour milk for 2
years. I kid you not! For my students, I had them put the "mud" on
masonite. It was dry by the next day. Then we tempera painted and
gold leafed the stuff. It lasts forever and the students were
thrilled to use "faux-painting" techniques.