A coloured spot. (Merriam – Webster)
A surface having coloured spots or blotches. (Merriam – Webster)
The arrangement of such spots or blotches on a surface. (Merriam – Webster)
Mark with spots or smears of colour. (Oxford Dictionaries)
An irregular arrangement of spots or patches of colour. (Oxford Dictionaries)
A spot or patch forming part of a mottled arrangement. (Oxford Dictionaries)
To colour with streaks or blotches of different shades. (Collins Dictionary)
Mottle definition and meaning.
Radiographic mottle: a comprehensive theory.