What may cause a supposedly clean green mixture to appear brownish?

Correct answer: There was red contamination in the brush or paint

The governing concept is complementary contamination in subtractive colour mixing. Green is generally produced from yellow and blue, but even a small amount of red from a dirty brush, water, or neighbouring paint can neutralise it because red is complementary to green. The mixture may then lose saturation and look muddy or brownish, so option D is correct. White paper, a pale layer, or a large palette does not independently create this brown cast.