Darken/lighten a hex colour

gistfile1.txt

function clamp(val, min, max) {
  return Math.max(min, Math.min(max, val));
}

function colorShift(hex, amount) {
  const colour = parseInt(hex.slice(1), 16);
  let r = (colour >> 16) + amount;
  let g = (colour & 0x0000FF) + amount;
  let b = ((colour >> 8) & 0x00FF) + amount;
  r = clamp(r, 0, 255);
  g = clamp(g, 0, 255);
  b = clamp(b, 0, 255);

  const out = `000000${(g | (b << 8) | (r << 16)).toString(16)}`.substr(-6);
  return `#${out}`;
}