Which primary colour should be increased to make violet appear cooler?

Correct answer: Blue

Violet is a mixture of red and blue. In the usual warm–cool classification, blue is the cooler primary colour, whereas red is warmer. Increasing the amount of blue shifts violet toward bluish violet and makes it appear cooler. Therefore option C is correct. More red would warm the violet, yellow is not one of its stated components, and orange is a secondary colour rather than a primary colour.