Which colour should be increased to make red-violet warmer?

Correct answer: Red

The governing concept is warm–cool colour bias in a mixture. Red-violet already contains red and violet; red is the warmer component, while violet contributes a cooler, bluer influence. Increasing the amount of red shifts the visual balance toward warmth, making option B correct. Increasing blue or adding green would cool or dull the mixture, and yellow is not the direct component needed for this red-violet adjustment.