The level aa requires a contrast ratio of at least 4.5:1 for normal. Aa (minimum contrast) and aaa (enhanced contrast). Tetradic color schemes are made form two couples of complementary colors in a rectangular shape on the color wheel.
Palette visualizer preview your colors on real designs for a better visual understanding. Instantly generate beautiful palettes by hitting the spacebar, or explore millions of popular ones. Regarding colors, the standard defines two levels of contrast ratio:
They are very versatile, and work best with one dominant color.