Oklch Skill

OKLCH Colors

OKLCH is a perceptually uniform color space where the numbers actually mean what you think they mean. Most color problems in CSS — broken palettes, failing contrast, hue drift — come from using color spaces that don't match how we see. OKLCH fixes the model so the tools work. To explore interactively, visit oklch.fyi.

Quick Reference

Why OKLCH

• Perceptual uniformity. Equal L steps = equal brightness. oklch(0.5 ...) is visually mid. HSL's lightness: 50% varies wildly by hue.
• Stable hue. HSL blue shifts toward purple as lightness changes. OKLCH hue stays constant across the full lightness range.
• Independent chroma. Chroma is an absolute measure of colorfulness that doesn't depend on lightness. HSL saturation does.
• Finite gamut. Not every oklch value maps to a displayable sRGB color. High-chroma values at certain hues will clip — gamut awareness is required.

OKLCH Syntax

oklch(L C H)
oklch(L C H / alpha)

oklch(0.637 0.237 25.331)
oklch(0.8 0.05 200 / 0.5)

Formatting: L and C use 3 decimal places, H uses up to 3. Drop trailing zeros. Format -0 as 0. Browser support: Baseline 2023, 96%+ global coverage.

Key Thresholds

Review Output Format

Always present color changes as a markdown table with Before and After columns. Include every color that was changed — not just a subset. Never list findings as separate "Before:" / "After:" lines outside of a table.

This keeps feedback scannable and diff-friendly. Each row is a self-contained change the developer can act on independently.

Common Mistakes

Reference Files

• color-conversion.md — Supported formats, conversion examples, bulk conversion rules, what to leave alone
• palette-generation.md — Scale convention, generation algorithm, multi-hue palettes, dark mode, why not HSL
• accessibility-contrast.md — APCA and WCAG 2 thresholds, fixing contrast with L, lightness gap guide, hue drift detection
• gamut-and-tailwind.md — sRGB vs P3, gamut clamping, CSS fallback patterns, Tailwind v4 @theme and migration

Accessibility & Contrast

Contrast is always measured between a foreground color (text, icon, or UI element) and the background color it sits on. When checking contrast, identify the background the element will be rendered against — typically the nearest parent's background color.

APCA thresholds (recommended)

APCA (Accessible Perceptual Contrast Algorithm) is more perceptually accurate than WCAG 2 and pairs naturally with oklch since both are grounded in perceptual lightness. Use APCA as the default.

Lc (Lightness Contrast) measures the perceived contrast between foreground and background. These are conservative approximations:

APCA's Lc value is signed — positive means light text on dark background, negative means dark text on light. Use the absolute value for threshold comparison.

WCAG 2 thresholds (for legal compliance)

WCAG 2 is still required when making formal WCAG 2.x conformance claims. It uses a luminance ratio that can be both too strict and too lenient depending on the color pair.

Fixing contrast with oklch

In hex/rgb, fixing contrast means trial and error across three channels. In oklch, contrast is controlled by lightness (L) alone — adjust the L distance between the foreground and its background:

/* Failing: text too close in lightness to its background */
color: oklch(0.65 0.2 250);       /* foreground */
background: oklch(0.75 0.05 250); /* background */

/* Fix: darken the text, keep C and H unchanged */
color: oklch(0.35 0.2 250);       /* foreground — more L distance */
background: oklch(0.75 0.05 250); /* background — unchanged */

Chroma has negligible effect on contrast — always adjust L, never C.

Quick lightness gap guide

• Light background (L > 0.85): foreground L should be below 0.45
• Dark background (L < 0.25): foreground L should be above 0.75

These are approximations — always verify with an actual contrast calculation.

Light vs dark color detection

A color is considered light when its oklch lightness exceeds 0.6:

if L > 0.6 → use dark text on this background
if L <= 0.6 → use light text on this background

Hue drift detection

To detect hue drift in an existing HSL palette:

1. Convert each step to oklch 2. Compare the H values across steps 3. If the hue spread is greater than 10°, the palette has visible drift

/* HSL blue ramp — hue shifts toward purple */
hsl(240, 80%, 20%)  →  oklch H ≈ 269
hsl(240, 80%, 50%)  →  oklch H ≈ 267
hsl(240, 80%, 90%)  →  oklch H ≈ 285  /* shifted 18° */

Color Conversion

When converting existing colors to oklch, convert the color values but leave everything else unchanged — don't change gradient interpolation, don't restructure the CSS.

Supported input formats

Conversion examples

/* Before */
color: #3b82f6;
background: #1e293b;
border-color: #e2e8f0;

/* After */
color: oklch(0.623 0.188 259.815);
background: oklch(0.279 0.037 260.031);
border-color: oklch(0.929 0.013 255.508);

/* Before */
color: rgb(59, 130, 246);
border: 1px solid rgba(0, 0, 0, 0.1);

/* After */
color: oklch(0.623 0.188 259.815);
border: 1px solid oklch(0 0 0 / 0.1);

Alpha uses the forward-slash syntax. Omit alpha when it's 1.

What to leave alone

• CSS keywords: currentColor, inherit, initial, unset, transparent
• Gradient interpolation methods — only convert the color stops, not the function itself
• Colors in third-party library configs that expect hex input

Bulk conversion

When converting an entire file:

1. Replace all hex colors with their oklch equivalents 2. Replace all rgb(), rgba(), hsl(), hsla() function calls 3. Leave gradient functions unchanged — only convert the color stops within them 4. Leave currentColor, inherit, transparent, and CSS keywords as-is 5. Preserve comments and formatting

Gamut Awareness & Tailwind v4

sRGB vs Display P3

Every sRGB color exists within Display P3, but not every P3 color exists within sRGB. Display P3 covers ~50% more colors.

Max chroma varies by lightness and hue

The gamut boundary is irregular. At L=0.5 in sRGB:

• Highest chroma: purple (H ≈ 285) at C ≈ 0.29
• Red-orange (H ≈ 0-30): C ≈ 0.20
• Lowest chroma: cyan (H ≈ 195) at C ≈ 0.09

The peak hue shifts with lightness. At L=0.7 magenta peaks, at L=0.9 green peaks. Cyan consistently has the lowest max chroma.

Gamut checking

If a color's chroma exceeds the maximum for its L/H/space, it clips. The fix: reduce chroma while keeping L and H constant.

/* Out of sRGB gamut */
oklch(0.7 0.35 150)

/* Clamped to max chroma */
oklch(0.7 0.22 150)

CSS fallback patterns

/* sRGB fallback for all browsers */
.accent {
  color: oklch(0.7 0.2 150);
}

/* P3 enhancement for wider gamut displays */
@media (color-gamut: p3) {
  .accent {
    color: oklch(0.7 0.3 150);
  }
}

For browsers without oklch support:

.accent {
  color: #4ade80;
}

@supports (color: oklch(0 0 0)) {
  .accent {
    color: oklch(0.7 0.2 150);
  }

  @media (color-gamut: p3) {
    .accent {
      color: oklch(0.7 0.3 150);
    }
  }
}

Tailwind v4

Tailwind CSS v4 defines its default palette in oklch. Custom themes should follow the same convention.

Custom color scale with @theme

@theme {
  --color-brand-50: oklch(0.971 0.012 250);
  --color-brand-100: oklch(0.932 0.028 250);
  --color-brand-200: oklch(0.882 0.048 250);
  --color-brand-300: oklch(0.812 0.078 250);
  --color-brand-400: oklch(0.722 0.148 250);
  --color-brand-500: oklch(0.623 0.188 250);
  --color-brand-600: oklch(0.535 0.168 250);
  --color-brand-700: oklch(0.445 0.138 250);
  --color-brand-800: oklch(0.362 0.108 250);
  --color-brand-900: oklch(0.289 0.078 250);
  --color-brand-950: oklch(0.215 0.048 250);
}

This gives you bg-brand-500, text-brand-200, etc. automatically.

Opacity modifiers

Tailwind's opacity modifier syntax works with oklch:

<div class="bg-brand-500/50"></div>
<!-- Compiles to: oklch(0.623 0.188 250 / 0.5) -->

Migrating existing themes

1. Convert all hex values in @theme to oklch 2. Replace any theme() references that used hex 3. Test dark mode — oklch values may look slightly different due to perceptual accuracy 4. Check for hardcoded hex in component code and convert those too

Palette Generation

The scale convention

Design system palettes use a numeric scale from 50 (lightest) to 950 (darkest). The standard labels by palette size:

9 steps is the standard default (matches Tailwind).

Algorithm

Given a base color with lightness (L), chroma percentage, and hue (H):

Step 1 — Lightness bounds:

delta = 0.4
minL = max(0.05, baseL - delta)
maxL = min(0.95, baseL + delta)

Lightness is clamped to [0.05, 0.95] to avoid pure black/white which have zero chroma.

Step 2 — Distribute lightness evenly from maxL (lightest, label 50) to minL (darkest, label 950).

Step 3 — Clamp chroma per step. Each lightness level has a different maximum chroma for a given hue and color space:

maxChroma = findMaxChroma(step[i].L, hue, colorSpace)
step[i].C = (chromaPercentage / 100) * maxChroma

This ensures every step is within gamut. High-chroma base colors will have lower chroma at the lightest and darkest ends — this is correct and expected.

CSS variable output

:root {
  --color-50: oklch(0.971 0.012 250);
  --color-100: oklch(0.932 0.028 250);
  --color-200: oklch(0.882 0.048 250);
  --color-300: oklch(0.812 0.078 250);
  --color-500: oklch(0.623 0.188 250);
  --color-700: oklch(0.445 0.138 250);
  --color-800: oklch(0.362 0.108 250);
  --color-900: oklch(0.289 0.078 250);
  --color-950: oklch(0.215 0.048 250);
}

Multi-hue palettes

When generating palettes for multiple hues, use the same lightness and chroma percentage for all. Same L guarantees equal perceived brightness. Same chroma percentage (not absolute chroma) guarantees equal vividness relative to each hue's maximum.

:root {
  /* Same L, same C% (80% of max) — different absolute C per hue */
  --blue-500: oklch(0.623 0.141 250);   /* 80% of max 0.176 */
  --green-500: oklch(0.623 0.157 145);  /* 80% of max 0.196 */
  --red-500: oklch(0.623 0.202 25);     /* 80% of max 0.253 */
}

Different hues have different max chroma at the same lightness. Using the same absolute C value across hues would make some appear more vivid than others.

Dark mode

Reverse the palette mapping so that the lightest step becomes the darkest and vice versa:

:root {
  --color-bg: var(--color-50);
  --color-text: var(--color-950);
}

.dark {
  --color-bg: var(--color-950);
  --color-text: var(--color-50);
}

This works because oklch's perceptual uniformity means equal L steps in both directions produce equally readable results.

Why not HSL palettes?

Hue drift: hsl(240, 80%, 20%) and hsl(240, 80%, 90%) are not the same perceptual hue. The light variant shifts ~18° toward purple. OKLCH hue is stable.

Brightness inconsistency: hsl(60, 100%, 50%) (yellow) and hsl(240, 100%, 50%) (blue) have the same HSL lightness but wildly different perceived brightness.