Which precaution is most important for producing clean secondary colours?

Correct answer: Use a clean brush and palette

The governing concept is controlled colour mixing. Secondary colours are produced by mixing two primary colours, so the brush and palette must be free from traces of other pigments. Unwanted residue can alter the mixture and make it dull or muddy. Mixing all three primaries or adding black does not ensure a clean secondary colour, and dirty water increases contamination. Therefore, option A is correct.