A painter wants to make green warmer. Which change is most appropriate?

Correct answer: Increasing yellow in it

The governing concept is relative temperature within a hue family. Green becomes warmer when it shifts toward yellow-green, because yellow is warmer than blue on the usual colour-temperature scale. Increasing yellow produces that shift. More blue would create a cooler blue-green, violet would not provide the clearest warm direction, and black mainly reduces value and saturation rather than reliably warming the hue.