What is the most direct way to change the temperature of violet?

Correct answer: Change the dominance of red or blue

The governing concept is temperature bias within a mixed hue. Violet contains red and blue; increasing the red component generally makes it warmer, while increasing the blue component makes it cooler. White and black mainly create tints and shades by changing value, although context can affect appearance. A grey background may alter perception but does not directly change the violet mixture. Hence option A is correct.