For what purpose can limited mixing of complementary colours be useful?

Correct answer: To slightly reduce colour intensity

The governing concept is complementary colour interaction. Colours opposite each other on the colour wheel tend to reduce one another’s intensity when mixed, especially in paint. Adding a small amount of the complementary colour can therefore soften an overly vivid mixture and move it toward a muted neutral. Option A is correct. It does not make the colour white, wet the paper, or shape the brush tip; those actions require different materials or techniques.