If the expected green mixture becomes muddy, what is a likely cause?

Correct answer: A trace of red was present in the brush or paint

The governing concept is colour contamination during subtractive mixing. Clean yellow and blue normally produce green, although their ratio can change the shade. A trace of red, often left in a brush, palette, or paint container, partially neutralises green because red is opposite green on the colour wheel, making the mixture dull or muddy. Therefore option B identifies the most likely cause; options C and D mainly affect warmth or intensity, not necessarily muddiness.