What should be added to make an overly red red-orange mixture more orange?

Correct answer: A little yellow

Red-orange is produced by combining red and yellow. When the mixture appears too red, the governing principle is to increase the proportion of its yellow component. Adding a little yellow gradually shifts the hue toward orange without changing it abruptly. Blue or green may dull the mixture, and black mainly lowers its value by darkening it rather than making it more orange. Thus, option C is correct.