After many workshops and experiments, you eventually end up with a tone of little watercolor tubes lying around.  I recently hit on a solution that I really like.  I dug up an old butcher’s pans that I used for puddles (Janet Rogers).  They are metal, which is important.

I then ordered a ton of half-pans and quarter-pans from Amazon.  And magnetic stripping, which I glued to the bottom of each pan.

Marked each pan with the color and manufacturer, and filled it up.

Voila! Before I start each painting, I set up my usual colors, and add in any that might work work that particular work.

All of the rest stay in a second pan.