What is a likely cause if green mixed from yellow and blue becomes muddy?

Correct answer: Contamination with red

The governing concept is colour contamination and complementary relationships. Green is made from yellow and blue, while red is its complementary colour; even a small amount of red can reduce green’s brightness and make the mixture look muddy. More yellow would shift it toward yellow-green, and more blue would shift it toward blue-green. Excess clean water changes transparency or strength rather than adding a muddy third pigment. Thus, option A is correct.