Which precaution is most necessary to maintain the purity of primary colours while mixing?

Correct answer: Use a clean brush or knife for each colour

The governing studio-practice concept is preventing accidental contamination during pigment mixing. A clean brush or palette knife keeps traces of another colour from entering the paint and changing its hue, value, or intensity. Thus option A is correct. One shared unclean tool can muddy colours; excessive water changes consistency rather than preserving purity, and a dusty palette introduces unwanted particles.