What result may commonly occur when complementary colours are mixed excessively?

Correct answer: A dull or brownish colour

Complementary colours lie opposite each other on the colour wheel. When they are mixed in excessive or poorly controlled amounts, their opposing colour properties reduce saturation and absorb or neutralise much of the visual intensity, commonly producing a muddy, dull, greyish, or brownish result. They do not normally create bright white, automatically become a primary colour, or become transparent. Thus, D is correct.