Check WCAG 2 contrast for text/background color pairs and say PASS or FAIL for each, with the exact ratio and, for failures, a nearby passing color. Also gives the signed APCA Lc (APCA-W3 0.98G-4g: positive for dark text on light, negative for light on dark). Use whenever you must know if colors are readable or accessible. Handles translucent colors. Pass foreground+background, or pairs for many.

Make a text color readable: returns the closest color (same hue) that reaches the target contrast ratio on the given background. Use after a contrast check fails, or to pick an accessible text color. The background stays fixed. Returns one hex color and its verified ratio.

Build a finished color palette with verified accessibility. kind "ramp": an 11-step tonal scale of a color (lightest to darkest, the seed is one step, evenly spaced). kind "theme": light and dark UI tokens (background, surface, surface-raised, text, text-muted, border, primary, on-primary) from a brand color, with every text contrast rule checked. kind "categorical": N clearly different, color-blind-safe colors for charts. kind "diverging": a scale from color through a light neutral to color2, with symmetric lightness. Returns hex values ready to use.

Find colors that satisfy your rules in one call. Combine any of: hue band (or the hue family of a color), chroma range, lightness range, contrast against one or more backgrounds (against), and for several colors a minimum lightness gap, hue gap or color difference (optionally color-blind safe). Returns verified hex colors with the most margin, or says which rule cannot be met.

Text over a photo: find the smallest overlay (scrim) opacity at which the text reaches a contrast ratio on every worst-case region of the image. Painted like CSS, on 8-bit channels. Returns the opacity and the ratio on each region.

Scan CSS for color problems and return exact findings: "text-contrast-aa" (color on background below 4.5:1), "nontext-contrast-3" (border-color on background below 3:1) and "near-duplicate-token" (two :root custom-property colors almost identical, ΔE00 under 2). Resolves var(--x) from :root. Use for any stylesheet or design review.

Validate color strings and describe each: hex, rgb, hsl, oklch, closest CSS name, how light it is, whether it fits sRGB, and contrast against white and black (so you can pick black or white text). Use to check that an input is a real color, or to understand an unfamiliar color. Bad inputs are reported per color, with a suggestion.

Convert colors to another format: hex, rgb, hsl, hwb, hsv, oklch, oklab, CSS lab/lch (D50), lab-d65/lch-d65 (CIELAB, D65, no adaptation), xyz (D65), linear (linear sRGB), p3, rec2020 (CSS, 2.4 gamma), rec2020-oetf (BT.2020 transfer) or rec2020-linear. Reads any of them as input. Handles lists. A color outside sRGB (for hex/rgb/hsl/hwb) is mapped in by lowering chroma only, and the result says so.

Measure how different colors look from a reference: CIEDE2000 (ΔE00, default), CIE94, CIE76 or Oklab distance, or all of them at once. Returns the nearest candidate and a full ranking, closest first. Use for 'which color is closest', 'are these the same color', or matching to a palette. Under 1 is not noticeable; over 10 is very different.

Mix colors or build a gradient between 2+ colors. Returns evenly spaced hex colors with both endpoints included, interpolated in OKLCH so midpoints stay clean (no gray mud). Set at for one mixed color (0.5 = even mix). Always in sRGB gamut.

Make colors lighter, darker, more or less colorful, or rotate the hue, by exact OKLCH amounts. Changes only the property you name; the result is always a valid sRGB hex (chroma is lowered if the color would leave the gamut). Examples: {property:"lightness",mode:"add",amount:-0.1}; {property:"hue",mode:"add",amount:40}; {property:"chroma",mode:"multiply",amount:0.6}.

Generate a color scheme from a base color by rotating its OKLCH hue (complementary, split-complementary, triadic, tetradic, analogous) while keeping lightness and chroma matched. Returns hex colors with their hue offsets; the base is included at offset 0. OKLCH hue steps look even, unlike HSL angles.

Simulate how colors look with color blindness (Machado 2009 model) and, for 2+ colors, measure whether the set stays distinguishable: returns the weakest pair per vision type. Use to check chart or status colors, or to find out if red/green pairs will collide. To BUILD a color-blind-safe set use colors_palette with kind "categorical".