A palette ( /ˈpælɨt/), in the original sense of the word, is a rigid, flat surface on which a painter arranges and mixes paints. A palette is usually made of wood, plastic, ceramic, or other hard, inert, nonporous material, and can vary greatly in size and shape. The most commonly known type of painter's palette is made of a thin wood board designed to be held in the artist's hand and rest on the artist's arm.
From the original, literal sense above came a figurative sense by extension, referring to a selection of colors, as used in a specific art object or in a group of works comprising a visual style. This second, figurative sense is the one extended in the digital era to the computing senses of "palette".
A wet palette is a sealable container with a layer of absorbent material (such as tissue paper or sponge) that can be soaked with water and a semi-permeable membrane (such as parchment, greaseproof paper or baking paper (silicone paper)) over that. The paint sits on the membrane and is kept wet by osmosis. Wet palettes can be bought, but are easily made.