bash function wrapper around a node script to check color contrast.

contrast.sh

# calculate the WCAG contrast ratio between a single hex color and white, or
# between two hex color values
contrast () {
  if [ -x "$(command -v node)" ]; then
    node -e "function hexToRgb(hex) {
      const shorthandRegex = /^#?([a-f\d])([a-f\d])([a-f\d])$/i;
      hex = hex.replace(
        shorthandRegex,
        (_m, r, g, b) => r + r + g + g + b + b
      );

      const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
      return result
        ? {
            r: parseInt(result[1], 16),
            g: parseInt(result[2], 16),
            b: parseInt(result[3], 16)
          }
        : undefined;
    }

    function luminance(r, g, b) {
      const a = [r, g, b].map((v) => {
        v /= 255;
        return v <= 0.03928 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, 2.4);
      });
      return a[0] * 0.2126 + a[1] * 0.7152 + a[2] * 0.0722;
    }

    const contrast = (c1, c2 = 'ffffff') => {
      if (!c1.match(/^[a-fA-F0-9]{6}$/)) console.log('First number not a hex color value')
      if (!c2.match(/^[a-fA-F0-9]{6}$/)) console.log('Second number not a hex color value')
      const rgb1 = hexToRgb(c1);
      const rgb2 = hexToRgb(c2);
      if (rgb1 && rgb2) {
        const c1luminance = luminance(rgb1.r, rgb1.g, rgb1.b);
        const c2luminance = luminance(rgb2.r, rgb2.g, rgb2.b);

        const ratio =
          c1luminance > c2luminance
            ? (c2luminance + 0.05) / (c1luminance + 0.05)
            : (c1luminance + 0.05) / (c2luminance + 0.05);

        return parseFloat((1 / ratio).toFixed(2));
      }
    };
    console.log(contrast('$1', '${2:-ffffff}') || '');
    "
  else
    echo "Node not installed"
  fi
}