If green made from yellow and blue is duller than expected what should be the first correction?

Correct answer: Make a new test with clean yellow and blue using a clean brush

The governing concept is pigment cleanliness and colour bias during mixing. A dull green may result from a contaminated brush, dirty palette, or pigments that already contain a complementary bias. The most reliable first step is therefore option C: repeat the test with clean tools and fresh, known yellow and blue before changing the formula. Black or white changes value, and red can muddy the mixture, so neither diagnoses the original problem.