Yellow and blue produced a greyish colour instead of bright green. What is another likely cause?

Correct answer: Red contamination in the mixture

The governing concept is complementary-colour neutralisation. Red is complementary to green, so even a small red residue on the brush or palette can reduce the saturation of a yellow–blue mixture and make it appear greyish or muddy. Clean water, a white palette, or thick paper does not introduce a colour that neutralises green. Therefore, option A is the correct cause; wash tools carefully before mixing bright colours.