Daler Rowney Stay Wet Palettes will keep acrylic paints wet for a longer working time, even with the lid removed. The palette works by osmosis - moisture evaporating from the surface is replaced by water from the dampened reservoir paper beneath the semi-permeable membrane, which forms the working surface of the Stay Wet Palette. The acrylic paint does not travel across the membrane, so stays on the top of the palette surface.
Colours can be left when not working by replacing the transparent lid - this will reduce evaporation allowing colours to remain workable for days. By adding additional water to the membrane, this can be extended for longer periods.