If colour mixtures repeatedly become muddy which habit should be corrected first?

Correct answer: Clean the brush and palette before every new mixture

The governing concept is prevention of unwanted pigment contamination. Residual paint on a brush or palette can enter the next mixture, adding an unintended hue and lowering its clarity. Therefore option D should be corrected first: clean or rinse the brush and remove contaminated paint before beginning each new mixture. Labelling samples, changing palette size, or rotating the paper does not remove stray pigment and cannot directly prevent muddiness.