Which colour is useful in a small amount to cool a warm brown?

Correct answer: Blue

The governing concept is colour temperature adjustment through controlled pigment mixing. A warm brown commonly has a red or orange bias, and blue is classified as a cool colour; therefore, a very small amount of blue can reduce the brown’s warmth. It should be added gradually because too much blue may make the mixture muddy or strongly cool. Yellow, orange, and red are warm choices and would generally increase warmth rather than reduce it.