The Collaborative International Dictionary of English v.0.48:

Palette \Pal"ette\, n. [See Pallet a thin board.]
   [1913 Webster]
   1. (Paint.) A thin, oval or square board, or tablet, with a
      thumb hole at one end for holding it, on which a painter
      lays and mixes his pigments. Hence, any other object,
      usually one with a flat surface, used for the same
      purpose. [Written also pallet.]
      [1913 Webster]

   2. Hence: The complete set of colors used by an artist or
      other person in creating an image, in any medium. The
      meaning of this term has been extended in modern times to
      include the set of colors used in a particular computer
      application, or the complete set of of colors available in
      computer displays or printing techniques.
      [PJC]

   3. Hence: The complete range of resources and techniques used
      in any art, such as music.
      [PJC]

   4. (Anc. Armor) One of the plates covering the points of
      junction at the bend of the shoulders and elbows.
      --Fairholt.
      [1913 Webster]

   5. (Mech.) A breastplate for a breast drill.
      [1913 Webster]

   Palette knife, a knife with a very flexible steel blade and
      no cutting edge, rounded at the end, used by painters to
      mix colors on the grinding slab or palette.

   To set the palette (Paint.), to lay upon it the required
      pigments in a certain order, according to the intended use
      of them in a picture. --Fairholt.
      [1913 Webster]

WordNet (r) 3.0 (2006):

palette knife
    n 1: a spatula used by artists for mixing or applying or
         scraping off oil paints