OKLCH
Also known as: OKLCH color space, oklch() CSS color function
OKLCH is a perceptually uniform color space — built on the OKLab perceptual model — that lets designers reason about colors using lightness, chroma, and hue values where equal numeric changes correspond to equal perceived changes. Modern CSS exposes it via the oklch() color function.
Overview
Older color spaces have a perceptual problem. In HSL, two colors with the same numeric lightness can look very different — yellow at L=50% reads as much lighter than blue at L=50%. That makes it hard to build palettes where every step feels evenly spaced. OKLCH fixes this: a lightness ramp from L=0.2 to L=0.9 in OKLCH actually steps evenly through perceived brightness.
CSS Color Module Level 4 added oklch() so designers can specify colors directly in this space. Browsers do the conversion to the underlying display profile. The shape is oklch(L C H / A) where L is lightness (0–1), C is chroma (0 to roughly 0.4), H is hue in degrees (0–360), and A is optional alpha.