The Rowney Stay Wet palette works by osmosis. Acrylic colours dry solely by evaporation but when applied to the membrane of a Rowney Stay Wet palette the water lost by evaporation is replaced by the moisture penetrating through the semi permeable membrane from the reservoir paper.
This enables you to paint all day with the colours uncovered. When not in use cover the colours with the transparent lid as this will reduce evaporation and leave your acrylic colours moist and workable for days. Adding a little water to the reservoir will keep the colours moist and workable for weeks.