It is my understanding that gouache is like opaque
watercolor; here's what wiki says:
http://en.wikipedia.org/wiki/Gouache. Traditional
tempera paint was a precursor to oils, an emulsion
with pigment and egg for binder. Today's commercial
"tempera" paint, or poster paint, is related to
gouache in that it is pigment combined with binder and
some substance to make it opaque.