Why fret over keeping the paint "fresh", when you could just pour it into the trays and let it dry? Tempera paint applies just fine by simply adding water to the brush before painting, just as you would with tray watercolors. By the way, they sell temperas in cakes that are used in precisely that way. Unless you are working on a project that somehow requires liquid tempera, that is the course I would take. I rarely use liquid tempera anymore, preferring the cake forms for convenience and longevity. If I need liquid paint for, say paper mache or sculptures, I use liquid
acrylic, which is cheaper, more color-fast and plain better than tempera. Hope this helps.