Which colour should be increased if red-violet has become too blue?

Correct answer: Red

The governing concept is corrective colour mixing: a mixed hue is adjusted by adding a small amount of the component that has become relatively weak. Red-violet contains a strong red component together with violet, which itself contains blue. If the sample looks too blue, adding red shifts its balance back toward red-violet. Green and yellow would introduce unwanted hue changes, while white would mainly reduce saturation and lighten the value rather than correct the blue bias. Therefore, option B is correct.