Which colour can be added in a small amount to reduce the intensity of bright red?

Correct answer: Green

The governing concept is the complementary relationship on the colour wheel. Green is complementary to red, so a small amount of green counterbalances red’s chromatic intensity and makes it appear more muted. Add it gradually because too much green can shift the mixture toward a dull brown or grey. Orange and yellow are neighbouring warm colours and may intensify or warm the red, while pink is a lighter red rather than its complement. Therefore, option C is correct.