Why may a red and yellow mixture become brown instead of clean orange?

Correct answer: Blue entered the mixture

The governing concept is that a clean secondary colour requires only its two component primaries. Red and yellow normally produce orange, but accidental blue introduces the third primary. Together, red, yellow, and blue absorb or neutralise more of the visible colour, making the mixture dull brown rather than clear orange. A larger amount of either red or yellow mainly changes the orange’s bias, and a clean palette prevents contamination. Therefore, option A is correct.