The complementary colour of a primary colour (magenta, cyan blue, or yellow) is the colour you get by mixing the other two primary colours. So the complementary colour of magenta is green, of cyan blue is red, and of yellow is purple.
When placed next to each other, complementary colours make each other appear brighter, more intense. The shadow of an object will also contain its complementary colour, for example the shadow of a green apple will contain some red. Green Apple, by Debra Sisson
The colour triangle makes it easy to remember: the three primary colours are in the corners. The colour you get by mixing two primaries is between them (magenta and yellow make red; magenta and cyan blue make purple; yellow and cyan blue make green). The complementary colour of a primary colour is the colour opposite it (green is the complementary of magenta, red for cyan blue, and purple for yellow).
If you mix complementary colours with one another, you get a tertiary colour, particularly browns (rather than grays).
Activities1. The Artist’s Toolkit: Complementary Colours. (you’ll need earphones to listen to it).2. Painting with complementary colours: André Derain. Boats at Collioure Harbour Charing Cross Bridge