Source : Webster's Revised Unabridged Dictionary (1913)

Water color \Wa"ter col`or\ (Paint.)
   1. A color ground with water and gum or other glutinous
      medium; a color the vehicle of which is water; -- so
      called in distinction from {oil color}.

   Note: It preserves its consistency when dried in a solid
         cake, which is used by rubbing off a portion on a
         moistened palette. Moist water colors are water colors
         kept in a semifluid or pasty state in little metal
         tubes or pans.

   2. A picture painted with such colors.