What is the main change when black is added to green?

Correct answer: Green will become darker

Adding black generally lowers the value, or lightness, of green, so the mixture appears darker. In colour theory this darker version is called a shade. Black does not change green into red or yellow by itself, because those hues require different pigments; adding white would move green in the lighter direction and create a tint. Thus the main expected effect is that green becomes darker.