random dark color - javascript

randomDarkColor.js

function randomDarkColor() {
  var lum = -0.35;
  var hex = String('#' + Math.random().toString(16).slice(2, 8).toUpperCase()).replace(/[^0-9a-f]/gi, '');
  if (hex.length < 6) hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
  var c, i, rgb = '#';
  for (i = 0; i < 3; i++) {
    c = parseInt(hex.substr(i * 2, 2), 16);
    c = Math.round(Math.min(Math.max(0, c + (c * lum)), 255)).toString(16);
    rgb += ('00' + c).substr(c.length);
  }
  return rgb;
}