If you want to be able to paint over a few days with your colours mixed, here's what I did.
I bought an A4 size rigid plastic document carrier which has the bottom part half inch, and the top quarter inch and fastens with two clips. I lined it with gardening felt (for protecting your plants over the winter), sprayed it well with water and then put a piece of baking parchment over the top.
Squeeze out your paints and away you go!! When you've finished, close the clips and I can guarantee that next time you want to paint, the paints will be as fresh as when you laid them out.
Check that the felt is wet from time to time.