Which colour can be useful in a small amount to dull red-orange?

Correct answer: Blue-green

The governing concept is complementary colour mixing. On a twelve-part colour wheel, blue-green is approximately opposite red-orange, so a very small amount of it can neutralise or dull the red-orange intensity. The quantity must be controlled because too much mixing may produce a muddy colour. Yellow-orange and yellow-green are neighbouring colours, while red-violet does not provide the closest complement.