Hue

Hue is the one of the three colour attributes that answers “which colour is it”. Every model that separates the attributes puts hue on a circle, because the family of colour names has no ends: keep walking past violet and you arrive back at red. That is why hue is written as an angle rather than an amount. hsl(210 80% 50%), hwb(210 20% 10%) and oklch(0.6 0.15 210) all say “the hue at 210 degrees” and differ only in what they do with the rest.

The angles are not portable between models. HSL derives its wheel from the RGB cube, so its hue numbers are whatever that geometry produces; oklch derives its wheel from a perceptual space, so the same name lands at a different number, and both differ again from the red, yellow, blue wheel taught in art classes. Two further traps come with the territory. Equal steps of hue are not equal perceptual steps, which is why a spectrum sliced into twelve even angles gives four greens and one orange. And hue does not carry lightness with it: holding HSL lightness at 50 percent while rotating the hue produces a yellow that glares and a blue that goes murky, because HSL’s lightness is arithmetic rather than perceptual. Rotating hue in oklch is the version that behaves.

In interface work hue rotation is how one curve becomes a whole palette. Fix the lightness and chroma of a colour ramp, change only the angle, and step 600 stays step 600 whether the product is indigo or amber. Hue is also how status is conventionally signalled, red for danger and green for success, which is exactly why it must never be the only signal: the most common colour vision deficiencies compress the red to green part of the wheel, so a hue difference that is obvious to one reader is invisible to another. Pair it with a shape, a label, or a difference in lightness.