COVE

The use of a palette when painting becmae popular in the 15th century when the use of oil paint as a medium grew. Oil paint is made by mixing pigment in with oil, commonly linseed oil. Originally oil paints were only made originally in a few colors, meaning artisit would have to mix small amounts of these colors on thier own to make new shades. This mixing was done on a wooden palette that was mobile and easy to bring around the studio with them. 

Source: https://en.wikipedia.org/wiki/Oil_painting

Image source: https://commons.wikimedia.org/wiki/File:DeltaIIndigo_Logo_Carr%C3%A9.jpg