Complementary, analogous and triadic colours

On the screen colour wheel, the complement of blue at 240 degrees is yellow at 60 degrees, while a painter's wheel puts orange across from blue.

Harmonies are angles

Design textbooks describe a handful of colour schemes that tend to look balanced. Once colours are placed on a hue circle, each scheme is just a set of angles from a starting hue. Complementary colours are directly opposite, 180 degrees apart. Analogous colours sit next to each other; the model uses 30 degrees on either side. A triadic scheme uses three hues evenly spaced, 120 degrees apart, so they form an equilateral triangle. A split complementary scheme takes the complement and replaces it with its two neighbours, here 150 and 210 degrees from the start.

The LibreTexts drawing course gives the character of each scheme. Complementary pairs give strong contrast and draw attention, which is why designers use them for things that must stand out. Analogous schemes look calm and unified and are common in nature, such as the greens, yellows and oranges of a meadow. Triadic schemes are lively but still balanced. Split complementary schemes keep much of the contrast of a complementary pair while being easier to live with. The University of Delaware library guide adds a practical rule of thumb for how much of each colour to use: about 60% for a dominant colour, 30% for a secondary colour and 10% for an accent.

Which wheel?

Here is a trap. There is more than one colour wheel. The HSL wheel in this course is built from light, with red, green and blue as primaries. On it, red at 0 degrees is opposite cyan at 180, green is opposite magenta, and blue is opposite yellow. Many art classes use a painter's wheel built on red, yellow and blue pigments. On that wheel, red sits opposite green, yellow opposite purple, and blue opposite orange, the pairs listed in the LibreTexts course. Neither wheel is wrong. They answer different questions. The light wheel tells you which two lights add up to white. The painter's wheel is a long tradition for mixing paint. When you build a palette for a screen, the HSL angles are the ones your code will use, so check which wheel a tutorial is talking about.

A harmony fixes the hues, not the lightness. Three triadic hues at full saturation and 50% lightness are loud. In practice designers keep the angles and change saturation and lightness: one hue strong, the others pale or dark. Try it in the model. Pick a triad, then lower the saturation, and notice that the triangle shrinks towards the grey axis while keeping its shape.

Two more schemes appear in most textbooks. A monochromatic scheme uses a single hue in several lighter and darker versions, which the RMIT colour theory text calls tints and shades. In HSL terms, the hue stays fixed and only lightness and saturation move, so in the model it is a vertical line on the upright slice. A tetradic scheme uses four hues that form a rectangle on the wheel, two complementary pairs together; when the four are evenly spaced, 90 degrees apart, they form a square. Four hues are hard to balance, so most guides suggest letting one of them lead and using the others in small amounts.