What should be added if yellow-green appears more blue than expected?

Correct answer: A little yellow

The governing concept is colour adjustment within a secondary hue. Yellow-green is made by combining yellow and green; if the mixture appears too blue or too cool, adding a small amount of yellow shifts it toward the intended yellow-green balance. Option A is therefore correct. Red would introduce a different hue, black would reduce lightness, and violet would usually make the mixture still cooler or darker.