Which process is best for making a clean green in watercolour?

Correct answer: Mix blue and yellow in controlled amounts with a clean brush

The governing colour-theory concept is that green is a secondary colour produced by mixing blue and yellow. In watercolour, a clean brush, clean water, and controlled quantities prevent unwanted pigment contamination, so the mixture remains bright and clear. Thus, option A is correct. Red and green tend to neutralise each other, while black and white make grey; mixing all three primaries commonly produces a dull neutral colour. The quality of the result also depends on using fresh, uncontaminated pigment.