Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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