What is a possible reason if mixing yellow and blue produces a muddy green?

Correct answer: The brush or paint contains another colour

Colour mixing explains that clean yellow and blue pigments generally produce green. If the result is muddy rather than clear, an unwanted third colour may have entered through a dirty brush, contaminated palette, or impure paint. Red or brown contamination can reduce the green’s brightness and clarity. Therefore option B is correct. The fact that yellow and blue are primary colours is not itself a cause, while a clean palette and white paper do not create muddy pigment.