Which colour change will make violet warmer?

Correct answer: Increase red

The governing concept is colour bias within a hue. Violet can lean toward blue-violet, which is cooler, or red-violet, which is warmer. Increasing red shifts the mixture toward red-violet, so option C is correct. Increasing blue strengthens the cool bias; adding green moves the colour away from a warm violet relationship, while black mainly lowers value and does not make the hue warmer.