My students burn through paint like there's a never-ending supply, and whatever we do have gets mixed into a sickly everycolourmixedup by the kids who will NOT BE BOXED IN by the tyranny of my one brush for one pot regime.
Solution? Cheap, home-made paint made to order.
There are a million DIY fingerpaint recipes out there, so like I do with everthing, I took the good bits from all that I read and made one up. I put about a quarter cup of flour in a large jar with some hot water and shook it. Then I added about the same amount of white glue and shook again. Then I added more water and shook again. Once I divided it amongst my little containers, I added food colouring and mixed it. I used concentrated colouring so my colours are vibrant. Regular grocery store stuff would probably be more pastel. I'm assuming more flour would make it thicker and more opaque. Experiment! What's the worst that can happen?
Anyway, pretty colours, works well, and basically free.