What can be added to a neutral brown mixture to make it cooler?

Correct answer: A little blue

The governing concept is warm–cool colour temperature. Blue is generally perceived as cool, so adding a small amount shifts a neutral brown toward a cooler, less reddish appearance. Option B is correct, but the quantity must be controlled: too much blue may make the mixture grey, muddy, or excessively dark. Yellow, red, and orange generally push the mixture toward warmer rather than cooler results.