When doing smaller paintings I use the plastic containers (two) as my palettes as well, and when I'm finished for the day, I just close the lids tightly, and my colours are ready and waiting to continue using the next day, Of course, you do need to close the lids when stopping for your morning coffee etc as well.
If the paint does start drying too quickly, particularly in the Greek summer, a little spray with some water, will revive them if they're not already too dried out. This though, leads me to another thought for perhaps my next blog post, and that is Atelier Interactive Paints - stayed tuned.