Why should the brush be cleaned before making a secondary colour?

Correct answer: To keep the mixture clean

The governing concept is colour purity during mixing. A secondary colour is made by combining two primary colours, so paint left on the brush can add an unwanted third colour. That contamination changes the hue and may make the mixture dull or muddy. Therefore, cleaning the brush keeps the intended mixture pure. Making the brush heavy, drying paint, or polishing paper is unrelated to colour formation.