Which habit is most useful for making secondary colours clean and bright?

Correct answer: Use a clean brush and clean palette

Clean tools are essential for controlled colour mixing. A clean brush and palette prevent leftover paint from contaminating a new mixture, so the secondary colour remains clearer and brighter. A dirty brush introduces unwanted hues; mixing all three primaries often produces a duller, neutralised result; and adding black reduces lightness and brightness. Thus option B describes the best studio habit for clean secondary colours.