What should be added if green appears too blue and needs to become a more natural yellow-green?

Correct answer: Yellow

Green is commonly formed from blue and yellow. When a green mixture is too blue, it lacks sufficient yellow relative to the blue. Adding a small quantity of yellow shifts the hue toward yellow-green, making option D correct. Red and violet introduce opposing or additional blue-red influence and can dull the mixture, while black only lowers its value. The safest method is to add yellow in small stages and test the result.