Color wheel

The wheel exists because hue has no ends. Red runs to orange to yellow to green to blue to violet and then back to red, so the sensible way to lay the names out is a circle, and once they are on a circle the relationships become angles. Opposite is 180 degrees. A triad is three points 120 degrees apart. Analogous colours are neighbours within about 30 degrees. This is the entire value of the device: a colour scheme stops being a matter of taste you have to argue about and becomes a shape you can draw, move as one piece, and hand to someone else.

There is more than one wheel, and they disagree. The red, yellow, blue wheel taught in art rooms comes from mixing pigment, where yellow and blue make green. The red, green, blue wheel comes from mixing light, where the complement of red is cyan rather than green. And a wheel drawn from hsl hue angles is different again from one drawn from oklch hue angles, because the two derive their circles from different geometry. So “the complement of red” has at least three defensible answers, and a scheme copied off one wheel and rebuilt on another will not land in the same places. Say which wheel you are on.

A wheel is also only about hue, which is why schemes picked from one so often look wrong when they are built. Twelve evenly spaced angles will hand you four greens and one orange, because equal angular steps are not equal perceptual steps. Two opposite hues at full saturation will vibrate against each other rather than harmonize. The wheel picks the family; the work of making it usable is done afterward in lightness and chroma, which is where a colour ramp comes in. Treat the geometry as a starting hypothesis, not a result.