Which mixture is suitable for making blue darker and less vivid?

Correct answer: Add a little orange to blue

The governing concept is complementary mixing for controlled muting. Orange is opposite blue on the traditional RYB colour wheel, so a small amount of orange lowers blue’s saturation and can produce a darker, more natural-looking blue-neutral shade. It should be added sparingly because excess orange may turn the mixture brown. White would lighten blue, yellow would shift its hue toward green, and much red would strongly alter and neutralize it.