Why is it necessary to clean the brush before mixing colours while painting?

Correct answer: To avoid unwanted colour mixing

The governing painting technique is brush hygiene and colour control. Paint left on a brush can enter the next colour and alter its hue, value, or cleanliness, producing an unintended mixture. Cleaning the brush before changing or mixing colours keeps the result predictable and helps preserve clear, bright colour relationships. Thus option D is correct; cleaning does not dry paint, thicken paper, or shorten the brush.