If a poster-colour mixture appears lighter than expected after drying what should be done next time?

Correct answer: Keep the wet mixture slightly darker based on a dry sample

The governing concept is the difference between wet and dry colour value. If a test patch becomes lighter after drying, the next wet mixture should be adjusted slightly darker so its dried value reaches the intended result. Hence, option C is correct. Adding water or white generally makes the mixture lighter, while changing the brush does not correct a predictable value shift. A dry sample provides the most useful reference.