Which colour should be increased to make blue-violet appear cooler?

Correct answer: Blue

The governing concept is colour temperature: blue is generally classified as a cool colour, while red, yellow, and orange are warmer influences. Blue-violet already contains blue and violet; increasing blue strengthens its cool bias and makes the mixture appear cooler. Red or orange would push it toward warmth, while yellow would alter the hue without giving the intended cool emphasis. Therefore, option A is correct.