A thin flat board or tablet on which an artist lays and mixes colours. Also figurative.

a palette of different colours of paint, esp. as used by an artist; (now usually in extended use) a range or selection of colours from which one can…

A porcelain palette divided into compartments, usually with sloping surfaces, for mixing and holding water-colours.

technical. A slab having shallow sloping compartments or depressions for water-colours.