Which colour is useful for reducing the intensity of primary red on a traditional colour wheel?

Correct answer: Green

The governing concept is complementary colour contrast. On the traditional RYB colour wheel, green lies opposite red, so it is red’s complementary colour. A small amount of green can neutralise or reduce red’s intensity; too much may make the mixture muddy or brownish. Orange, violet and yellow are not the direct complement of red on this wheel. Therefore, option A is the most appropriate answer.