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// Adjust value of a HEX color
// jsfiddle.net/y2mgW
function colorLuminance(hex, lum) {
  // validate hex string
  hex = String(hex).replace(/[^0-9a-f]/gi, '');
  if (hex.length < 6) {
    hex = hex[0]+hex[0]+hex[1]+hex[1]+hex[2]+hex[2];
  }
  lum = lum || 0;
  // convert to decimal and change luminosity
  var rgb = '#', c, i;
  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;
}



console.log( 
  colorLuminance("#00ff00", -0.5) // = #008000
);