Which colour combines with the primary colour yellow to make green?

Correct answer: Blue

Yellow and blue combine to make green in the traditional paint colour model, so blue is correct. This is one of the three standard secondary-colour relationships: yellow plus blue gives green, red plus yellow gives orange, and red plus blue gives violet. Red would lead toward orange, while orange and violet are already secondary colours and are not the primary partner requested. The answer concerns pigment mixing, not light mixing.