Here’s a clear, explanatory text on the (also known as a double-complementary scheme): The Tetrad Color Scheme: Balanced Contrast A tetrad color scheme uses four colors arranged into two complementary pairs. On the standard color wheel, these four colors form a rectangle (or “tetrahedron”), with equal spacing between each pair. How It Works If you pick a base color (e.g., blue ), its complement ( orange ) is directly opposite. Then choose a second complementary pair (e.g., yellow and purple ). Together, blue, orange, yellow, and purple form a tetrad.