What long-term effect may occur if a dirty brush is dipped into a poster-colour container?

Correct answer: The original colour may gradually become contaminated

The governing concept is material hygiene and colour contamination. A dirty brush may carry traces of another colour, dust, or excess water into the container. Repeated contamination can gradually change the original hue and reduce its intended clarity. Hence, option A is correct. The paint will not necessarily become lighter, the container cannot expand because of the brush, and contamination does not stop drying automatically. Use a clean, suitably rinsed brush when taking colour from the original container.