What is the best way to prevent a mixed colour from becoming muddy?

Correct answer: Mix fewer colours with a clean brush

The governing concept is pigment purity during colour mixing. Using only a few compatible colours and a clean brush limits unwanted pigment contamination, so option C is correct. Mixing many colours or excessive complementary colours introduces competing pigments that absorb more light and often produce a dull, muddy result. Adding black to every mixture reduces brightness and cannot replace careful selection or a clean mixing tool.