Which precaution is most useful for keeping blue-violet clean?

Correct answer: Avoid contamination by yellow or orange

Blue-violet is most likely to lose its clean, vivid character when warm neighbouring or opposing pigments such as yellow or orange accidentally enter the mixture. Using a clean brush, palette area, and water container helps prevent this contamination. Therefore B is correct. Black, excess water, or mixing every colour together would not preserve hue purity and may reduce intensity or value control.