Which habit is necessary for making a clean green watercolour?

Correct answer: Use clean blue and yellow paints with a clean brush

The governing concept is controlled colour mixing. A clean green is commonly produced by combining clean blue and yellow pigment with a rinsed brush and reasonably clean water. Red contamination or dirty water introduces unwanted pigments, while mixing all three primary colours usually reduces saturation and produces a duller mixture. Therefore, option D describes the necessary studio habit and correct colour relationship.