Determine if a colour appears light or dark

gistfile1.js

var light_or_dark = function(hex) {
  var rgb = /^([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex),
      luma = Math.sqrt(
        0.299 * parseInt(rgb[1], 16) +
        0.587 * parseInt(rgb[2], 16) +
        0.144 * parseInt(rgb[3], 16)
      );

  return (luma > 13) ? "light" : "dark";
};

Useful for picking between light/dark text to go over a block colour.