What change is needed to turn yellow-green into blue-green?

Correct answer: Increase blue

The governing concept is hue shift through changing pigment proportion. Yellow-green contains a stronger yellow influence, while blue-green is green moved toward blue on the colour wheel. Increasing the amount of blue shifts the mixture in that direction, so option B is correct. Red would push the colour away from the target, whereas white mainly creates a lighter tint and black mainly creates a darker shade without producing the required blue-green hue.