Both grounds, tested

A palette designed on white and shipped on charcoal usually fails, because the mid-tone amber that read as warning on paper vanishes. Every value here is measured against both grounds, and the rows that miss the floor say so.


Tokens this needs: --muted-foreground, --status-critical
The status, chart, scale, state and direction names are an extension,
not a rename. shadcn has --destructive and five --chart-* and nothing
else in this territory.

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// ContrastLadder.tsx
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import { contrast } from "./contrast";

/** WCAG 2.2 SC 1.4.11. A chart element carries meaning, so it is non-text
 *  content and 3:1 is the floor, on every ground the chart will be shown on. */
const FLOOR = 3;

export type Step = { label: string; color: string };

/**
 * One ground, every value of the palette on it, and the ratio each one
 * actually reaches. The ground is a prop rather than the page background
 * because the point is to run this against the charcoal the chart ships on,
 * not the white it was designed on.
 */
export function ContrastLadder({ label, ground, steps, floor = FLOOR }: {
  label: string;
  /** The background the chart will be drawn on, as #rrggbb. */
  ground: string;
  steps: Step[];
  floor?: number;
}) {
  const rows = steps.map((s) => ({ ...s, ratio: contrast(s.color, ground) }));
  const passing = rows.filter((r) => r.ratio >= floor).length;

  return (
    <figure className="m-0">
      <div className="grid grid-cols-[auto_220px_auto] items-stretch gap-x-2">
        {/* The labels sit outside the ground, so they stay readable on charcoal. */}
        <div className="flex flex-col justify-around py-3 text-right text-[11px] tabular-nums text-muted-foreground">
          {rows.map((r) => <span key={r.label}>{r.label}</span>)}
        </div>
        <div className="flex flex-col gap-1.5 rounded-md p-3" style={{ background: ground }}>
          {rows.map((r) => <span key={r.label} className="h-7" style={{ background: r.color }} />)}
        </div>
        <div className="flex flex-col justify-around py-3 text-[11px] tabular-nums">
          {rows.map((r) => (
            <span key={r.label} className={r.ratio >= floor ? "text-muted-foreground" : "text-status-critical"}>
              {r.ratio.toFixed(1)}:1{r.ratio < floor && " below floor"}
            </span>
          ))}
        </div>
      </div>
      <figcaption className="mt-2 text-center text-[11px] text-muted-foreground">
        {label} · {passing} of {rows.length} reach {floor}:1
      </figcaption>
    </figure>
  );
}

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// contrast.ts
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/**
 * WCAG 2.2 contrast, the way SC 1.4.11 measures a non-text element against
 * its ground. Small on purpose: the check has to be cheap enough to run on
 * every colour against every ground, not once on the one someone remembered.
 */

const channel = (c: number) => (c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4);

/** Relative luminance of a #rrggbb colour, 0 for black and 1 for white. */
export function luminance(hex: string) {
  const n = parseInt(hex.slice(1), 16);
  const [r, g, b] = [n >> 16, (n >> 8) & 255, n & 255].map((v) => channel(v / 255));
  return 0.2126 * r + 0.7152 * g + 0.0722 * b;
}

/** Contrast ratio between two colours, 1 to 21. */
export function contrast(a: string, b: string) {
  const [hi, lo] = [luminance(a), luminance(b)].sort((x, y) => y - x);
  return (hi + 0.05) / (lo + 0.05);
}

/** A neutral grey at a given CIE L*, so a ladder can be stated in lightness. */
export function greyAtLightness(L: number) {
  const Y = L > 8 ? ((L + 16) / 116) ** 3 : L / 903.3;
  const c = Y <= 0.0031308 ? 12.92 * Y : 1.055 * Y ** (1 / 2.4) - 0.055;
  const h = Math.round(c * 255).toString(16).padStart(2, "0");
  return `#${h}${h}${h}`;
}

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// demo.tsx
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import { ContrastLadder, type Step } from "./ContrastLadder";
import { greyAtLightness } from "./contrast";

/**
 * Six greys stated in CIE L*, run against the two grounds the chart will
 * live on. On white the dark end passes; on charcoal only the light end
 * does, and the usable range is the rows that clear the floor.
 */
const STEPS: Step[] = [88, 72, 56, 40, 24, 8].map((L) => ({
  label: String(L).padStart(2, "0"),
  color: greyAtLightness(L),
}));

export default function Demo() {
  return (
    <div className="flex flex-wrap gap-6">
      <ContrastLadder label="On white" ground="#ffffff" steps={STEPS} />
      <ContrastLadder label="On charcoal" ground="#1c1c1e" steps={STEPS} />
    </div>
  );
}