What should be done in poster paint if the mixed colour becomes too dark?

Correct answer: Add a little white

The governing concept is value adjustment: adding white to a paint mixture raises its lightness and creates a lighter tint. If the poster colour is too dark, a small amount of white should be mixed in gradually, with testing after each addition. Adding black would make the problem worse, drying water does not reliably change the pigment value, and discarding the paint is unnecessary. Therefore, option B gives the most controlled correction.