Which action is most important for producing a clean secondary colour?

Correct answer: Use a clean brush and separate mixing area

The governing concept is colour purity during subtractive mixing. A secondary colour is made by combining two primary pigments, such as yellow and blue for green or red and yellow for orange. Residue from a third pigment, a dirty brush, or a contaminated mixing area introduces unwanted absorption and makes the result muddy. A clean brush and separate area prevent contamination. Therefore option B is correct; the other actions reduce control or add impurities.