For those who don't know what a "wet palette" is, it's essentially a closable tray with a layer of absorbent material soaked in water covered by a semi-permeable liner on which you put your water-based paints. Water from the lower layer can penetrate the liner by osmosis, keeping the paint moist and workable, for several days if required.
They can be purchased from artists supply stores, but I made myself a small one using s small, flat food storage container (with a lid), a half of a kitchen sponge and a small piece of silicone baking paper.. Total cost, about $3.00. This is sufficient to keep small amounts of Vallejo acrylics usable for up to a week - 10 days. It's invaluable when your painting time is short or rife with interruptions or distractions.