Complementary colours
Two colours opposite each other on the colour wheel, such as blue and orange.
Also: complements, opposite colours
Complementary colours sit directly across the wheel from each other: red and green, blue and orange, yellow and purple in the painter's version. Placed side by side they make each other look stronger, which is why orange lifebuoys show up against blue sea. Mixed together as paint they cancel out to a grey or brown, so a little of the complement is a useful way to dull a colour. The exact pairs depend on the wheel used. On a screen, the opposite of red is cyan, not green.