What is suitable to add if a violet mixture becomes too blue?

Correct answer: A little red

Violet is formed by mixing red and blue. If the mixture has too much blue, the governing colour-mixing principle is to add a small amount of the deficient component, red. Add it gradually and mix after each addition so the hue moves toward balanced violet. More blue increases the error, while yellow or green can reduce brightness and make the mixture dull or muddy. Therefore, option A is correct.