Which precaution is most necessary for obtaining a clean secondary colour sample?

Correct answer: Use a clean brush and palette

The governing concept is avoiding contamination during practical colour mixing. A clean brush and palette prevent traces of a previous colour from entering the new mixture, so the secondary colour remains clear and identifiable. Therefore, option B is correct. A dirty brush introduces an unwanted third colour; black darkens the sample, and mixing all three primaries produces a dull, muddy result rather than a clean secondary colour.