What precaution should be taken when mixing complementary colours to maintain cleanliness in watercolour?

Correct answer: Mix them sparingly and make a test swatch

Complementary colours sit opposite one another on the colour wheel and can neutralise each other quickly. If they are overmixed, their pigments often produce a dull grey or muddy mixture, so the practical precaution is to mix a small amount and test it on a swatch first. Scrubbing increases muddiness, white changes opacity, and soaking the paper does not control colour cleanliness.