Which colour should be added if a yellow-orange mixture appears too orange?

Correct answer: A little yellow

Yellow-orange lies between yellow and orange and contains both yellow and red influences. If it appears too orange, the governing mixing principle is to increase the yellow component carefully. A little yellow shifts the hue back toward yellow-orange. Red would make it still warmer and more orange, while blue or green can dull the mixture and move it away from the intended bright hue. Therefore, option D is correct.