The Collaborative International Dictionary of English v.0.48:

Mottle \Mot"tle\, v. t. [imp. & p. p. Mottled; p. pr. & vb. n.
   Mottling.] [From Mottled.]
   To mark with spots of different color, or shades of color, as
   if stained; to spot; to maculate.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

mottling \mottling\ n.
   1. the act of coloring with areas of different shades.
      [WordNet 1.5]

   2. A mottled pattern.
      [PJC]

WordNet (r) 3.0 (2006):

mottling
    n 1: the act of coloring with areas of different shades