Front Cover: "Starry Night", Painting by Vincent van Gogh, showing use of complementary colours. See the article for more details.
Back Cover: "Impression, Sunrise", Painting by Claude Monet, also showing use of complementary colours orange and blue.
Colour Wheel showing the three complementary colours cyan (C), magenta (M) and yellow (Y). The three primary colours blue, green and red are complementary to yellow, magenta and cyan respectively. All overlayed together gives black for paint or white for light.