What can be a common reason for not obtaining a clean violet?

Correct answer: Yellow contamination in the red or blue paint

The governing concept is maintaining purity when mixing a secondary colour. Violet is produced from red and blue, so yellow contamination in either paint adds a third primary colour. Since yellow is complementary to violet, it can reduce the mixture’s brightness and make the result muddy or brownish. A clean brush, separate palette spaces, and suitable proportions help produce cleaner violet; therefore option A identifies the actual problem.