Casein Paint Shelf Life
I do not think you have to worry about commercial casein paints in tubes unless you are leaving them alone for months. Doubt they would spoil, one site said you can refrigerate them if you wish, but do not freeze. 

Maybe you are thinking about egg casein which is mixed fresh from egg whites? Or other methods that start from powders? Those would go bad but the tubes seem to be stable.

"Casein paint tubes may harden a bit if they aren't handled frequently. Squeezing the tubes circulates the emulsion with the pigments, keeping them moist and pliable."

FAQ 1 - good overview of casein tubes. source of quote.

FAQ 2 - more focused on freshly mixed caseins

FAQ 3 - about underpainting with casein, talks about true casein spoiling though