Which blue is more suitable for creating a cool blue effect?

Correct answer: Green-biased blue

The governing concept is relative temperature within neighbouring hues. In the conventional warm–cool arrangement used in painting, a green-biased blue is generally treated as cooler than a violet-biased blue, which leans toward the warmer red-violet side. Adding red pushes blue toward violet, while greying it mainly reduces saturation rather than defining temperature. Therefore, among the listed choices, option B is the clearest choice for a cool blue effect.