If a mixed colour dries greyer than expected what is the most likely cause?

Correct answer: Excessive mixing of complementary or multiple colours

The governing concept is chromatic purity in colour mixing. Complementary colours contain opposing colour tendencies, so mixing too much of them neutralises the mixture and moves it toward grey or brown. Repeatedly combining several pigments has a similar muddying effect because more wavelengths are absorbed. Keeping colours separate, using clean water, and applying fewer strokes generally help preserve clarity rather than cause greyness. Hence, option A is the most likely cause.