I was always cleaning up the lids of my watercolor boxes. Seems like a tidy way to do things. Start fresh every day. Just wipe with damp paper towel. Done.
BUT- I observed friends who are better water color painters were leaving their mixing trays messy. hmmm... whats that about. I started to leave mine messy and discovered many interesting neutrals can be mixed with the dried colors. Mixing one neutral with another... what amazing colors. I think because they are more experienced they recognize those dried colors and the mixes. Perhaps they have a preferred palette and rely on certain colors.
Because I am using such a variety of references -i often try to match the palette to the ref.
I usually mix my colors on the paper wet into wet or by glazing over a dried color. Im working my way through this puzzle. I think i will try more of the tray mixing. Im agaist wasting good paint. I hated to toss those damp paper towels with the interesting colors after cleaning up. Lol.