Yellow was added to mute violet poster colour but the mixture became too grey. What was the error?

Correct answer: Too much complementary yellow was added

The governing colour-theory concept is complementary neutralisation. Yellow and violet are complementary colours, so mixing a small quantity of yellow into violet can reduce its intensity. However, adding too much of the complement pushes the mixture toward a dull neutral grey or brown. Option A is therefore correct. The lightness of the yellow, the dryness of the violet, and a clean palette do not by themselves explain excessive greying; the controlling error is quantity.