Why should a brush be clean before mixing colours?

Correct answer: To keep colours pure and clear

The governing practical principle is tool cleanliness in colour mixing. A brush carrying leftover paint can introduce an unwanted colour into the next mixture, changing its hue and making the result muddy or inaccurate. Therefore, option A is correct: a clean brush helps keep colours pure and clear. Cleaning a brush does not thicken paper, enlarge a drawing, or require the brush to remain permanently colourless; it simply prevents contamination between mixtures.