In a large sky wash with a mop brush the colour became dark at the top and unintentionally pale below. What is the likely cause?

Correct answer: The brush was not reloaded with an equally concentrated mixture while moving downward

The governing concept is consistent pigment concentration during a broad wash. If a mop brush carries most of its mixture at the beginning and is not reloaded with an equally strong mixture, the upper strokes receive more pigment and the lower strokes become pale. Therefore option D is correct. A flat sheet, a clean brush, or the colour name does not explain this concentration gradient.