Use oklch() and oklab() for perceptually uniform colour palettes

HSL and hex colours are defined in the sRGB colour space, which is not perceptually uniform — a 10 % lightness change looks dramatically different depending on the hue. This makes creating accessible, harmonious palettes by hand extremely difficult. oklch corrects this: adjusting the L channel produces the same perceived brightness change regardless of hue or chroma, making it far easier to build accessible colour ramps, dark-mode palettes, and consistent hover/active states.

Quick Reference

• oklch(L C H) — Lightness, Chroma, Hue — is the most practical perceptually uniform colour space for design systems
• Equal lightness steps in oklch look equal to the human eye; equal steps in hsl do not
• Define your colour palette tokens in oklch custom properties
• Use @media (color-gamut: p3) to serve wide-gamut colours to capable displays

Check

Check whether the CSS colour tokens use oklch() or a perceptually uniform colour space, or whether they use hsl/hex which may produce inconsistent perceived lightness across hues.

Fix

Convert the colour token palette from hsl/hex to oklch, ensuring the lightness (L) axis is consistent across all hues in the same shade step.

Explain

Explain what perceptual uniformity means in colour spaces, why oklch is better than hsl for design systems, and how the L, C, H channels map to human colour perception.

Code Review

Review CSS custom properties for colour tokens. Flag palette definitions in hsl or hex that use hardcoded values without a systematic lightness ramp, especially if they are intended to be interchangeable shade steps.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/css/color-oklch