What change is suitable for making a cool brown warmer?

Correct answer: Add a little red or orange

The governing concept is colour temperature. A cool brown often contains a noticeable blue bias, so adding a small quantity of a warm colour such as red or orange shifts the mixture toward warmth. The change should be gradual because a large amount may overpower the brown and produce an overly red or orange result. More blue would cool it further, while black and grey mainly reduce lightness or saturation instead of warming the hue.