What is a suitable way to create olive green with poster colour?

Correct answer: Add a little yellow and very little black to green

Olive green is a subdued, yellowish green. A practical poster-colour method is to begin with green, add a little yellow to make it warmer, and then add a very small amount of black to reduce brightness; option A is correct. The black must be added cautiously because it can overpower the mixture. More white makes a light tint, red and blue tend toward violet or a dark mixed colour, and white added to yellow creates a pale yellow tint.