What effect may occur when yellow is added to violet made from red and blue?

Correct answer: The colour may become muddy or dull

This question tests colour mixing and the effect of combining opposing colour components. Violet is made from red and blue; adding yellow introduces a third primary colour that partly counteracts the violet mixture. Instead of increasing purity, the combined pigments absorb more light and the result may lose brightness, becoming muddy, greyish, or dull. Therefore option C is correct. It will not normally become pure white or green.