Lighten/darken hex color

gistfile1.js

// shadeColor("#f0f0f0", -20) returns "#bdbdbd"

function shadeColor(color, percent) {
  var num = parseInt(color.slice(1), 16),
      amt = Math.round(2.55 * percent),
      R = (num >> 16) + amt,
      B = (num >> 8 & 0x00FF) + amt,
      G = (num & 0x0000FF) + amt;
  return "#" + (
             0x1000000 + (R < 255 ? R < 1 ? 0 : R : 255) * 0x10000
             + (B < 255 ? B < 1 ? 0 : B : 255) * 0x100
             + (G < 255 ? G < 1 ? 0 : G : 255)
          ).toString(16).slice(1);
}