Saturation

Saturation is the axis between grey and the purest version of a hue. At zero there is no hue left to name, only a neutral; at the top the colour is as intense as the model can express at that lightness. It is the knob a designer reaches for when a palette is the right colour but too loud, or right but too timid, and the one channel that can be turned all the way down without changing which colour something is meant to be.

The strict vocabulary is messier than the slider suggests. Colour science distinguishes chroma, colourfulness measured on its own, from saturation, colourfulness measured relative to how light the sample is: a dark navy and a pale sky blue can share a saturation figure while differing wildly in chroma. HSL uses the word loosely, and it pays for that. Its saturation does nothing at all near black or white, it interacts with its own lightness channel so that raising one visibly moves the other, and 100 percent means something different at every hue. oklch splits the job the useful way: chroma is an absolute amount that behaves the same wherever you are on the wheel, with the catch that how much of it a screen can actually show depends on the display’s gamut.

In practice saturation is budgeted by area. Small things can be intense, so accents, badges, and focus rings live near the top of the scale, while anything large enough to sit under text (page backgrounds, table fills, banner surfaces) is pulled far down, because a saturated field makes the type on it vibrate and tires the eye within a paragraph. Dark themes need the same colours desaturated again, since a fully saturated hue against a dark surface glows and smears. And the neutrals are where the interesting decision hides: a grey at exactly zero saturation is inert, so most systems keep a few percent of a hue in theirs, which is what colour temperature is about.