I am not an expert but I do know that certain colors even in acrylic paints can be toxic, (especially reds) so I would maybe start there. Red is a fairly toxic paint in painters acrylics, even doing paintings they are supposed to be used with caution, well, they used to, I have not kept up with that for years now.
All I'm really bringing up is to make sure to check to see how safe it is, that's all, some stuff can be scary.
I always thought that the woman from Goldfinger (James Bond) would end up crippled or something after they spray painted her, but years later I read that they did use a certain type of non-toxic paint on her, or at least that's what I read.
Although tempera paints sound like they may be non-toxic I am not so sure, they are mostly made with egg, I believe, so that may be fine but the tinting may not be non-toxic.
I would either check with these people or read their site, they are safety conscious and they are like hippies so they may be able to help out in one way or another.
The bad part is that you need a lot, that would automatically make me think, "cheap house paint!", heheh, but that may not be a good idea, although it's the stronger pigments in the paints that are more than likely the culprits in the toxicity department.
There was some guys that did this in the last few years in the Nude Forum that used large amounts of paint, I believe, I say, "believe" because I am unsure as to what they used, but they appeared to be using a thick latex looking paint, I sure might be wrong about that though. Maybe somebody else will remember which ones I mean.\