How should the amount of yellow be controlled when muting bright violet?

Correct answer: Add it gradually in very small amounts

The governing concept is complementary colour mixing. Yellow lies opposite violet on the colour wheel, so it can reduce violet’s saturation quickly and may produce a dull grey or muddy result if overused. Option C is correct: add a very small amount, mix thoroughly, observe the change, and repeat only if necessary. Equal quantities are too difficult to control, while refusing to add yellow or adding black first does not directly use the intended complementary correction.