This is mostly casein with just the addition of lemon yellow gouache paint I have in stock.
While working on this one, I was reminded of one of the reasons I went away from painting with gouache: the white disappears as it dries. So I ended up switching midway through to casein white.
The primary issue I’d been having with casein, prompting a return to gouache, is the lack of availability of a yellow that isn’t cadmium-based; I’m avoiding heavy metal pigments on principle.
Now that I’ve mixed gouache with casein, it seems to me that the gouache possibly mixes better than two caseins together. More experimenting is called for…