If yellow-green appears too green, what should be added to balance it?

Correct answer: Yellow

The governing concept is correcting a colour mixture by adjusting the proportion of its component colours. Yellow-green contains yellow and green; if it appears too green, adding yellow increases the yellow influence and moves the mixture back toward a balanced yellow-green. Thus option A is correct. Blue would make it cooler, while red or violet would introduce unrelated colour influences rather than restore the intended balance.