Each next value in an opaque colour scale is becoming muddy. Which correction will be most effective?

Correct answer: Use a new or thoroughly cleaned brush for each mixture

The governing principle is pigment cleanliness during opaque colour mixing. Residue left in a brush or palette contaminates the next mixture, so successive values lose clarity and become muddy. A clean or separately prepared brush prevents unwanted pigments from entering each value. A dirty container repeats the contamination, complementary colour generally reduces saturation further, and rubbing on paper cannot reliably correct a contaminated mixture. Thus A is correct.