What should be added if a lighter green is needed after mixing equal amounts of yellow and blue?

Correct answer: White colour

The governing colour-theory concept is tinting. Equal amounts of yellow and blue produce green, and adding white increases its lightness without changing the basic hue greatly, creating a lighter green tint. Black would produce a shade by darkening the mixture, while more blue would make the green bluer and cooler rather than simply lighter. Therefore, white is the correct choice.