Which contamination should be checked first if yellow-green appears muddy?

Correct answer: Unwanted red or violet contamination

Yellow-green is expected to look relatively clear and lively when its yellow and green components are clean. Red or violet contamination can introduce an opposing colour influence and partially neutralise the mixture, making it dull or muddy. A little extra yellow or green may change the hue bias but does not normally create muddiness by itself, while clean water is not a contaminating colour. Thus, the first practical check should be for red or violet traces on the brush, palette, or water, making option A correct.