In which situation can a colour mixture become muddy?

Correct answer: Mixing all three primary colours in uncontrolled amounts

The governing concept is controlled colour mixing. When all three traditional primary paints—red, yellow and blue—are mixed in uncontrolled amounts, several pigments absorb different wavelengths and the mixture loses clarity, often appearing brownish or muddy. Therefore option D is correct. A clean brush reduces contamination, keeping colours separate prevents mixing, and using a small amount alone does not cause muddiness.