If green made from yellow and blue is cooler than expected what should be done to warm it?

Correct answer: Increase the amount of yellow

The governing concept is that the proportions of component colours control a mixed colour's temperature. Yellow is commonly associated with warmth, while blue tends to make green cooler. Therefore, increasing yellow gradually shifts the green toward a warmer yellow-green; option A is correct. More blue would intensify the cool quality, and violet or black would darken or dull the mixture rather than warm it.