Calculate Lighter or Darker Hex Colors

color_luminance

  #
  # Examples:
  #     color_luminance("#69c", 0);		  # returns "#6699cc"
  #     color_luminance("6699CC", 0.2);	# "#7ab8f5" - 20% lighter
  #     color_luminance("69C", -0.5);	  # "#334d66" - 50% darker
  #     color_luminance("000", 1);		  # "#000000" - true black cannot be made lighter!
  #
  # based on code from
  # http://www.sitepoint.com/javascript-generate-lighter-darker-color/
  #
  def color_luminance(hex, lum)
    color = hex.strip.match(/\A#?([0-9a-f]*)\z/i)
    return hex if color.nil?
    color = color[1]
    if color.length < 6
      color = color[0] + color[0] + color[1] + color[1] +color[2] + color[2]
    end
    rgb = '#'
    (0..2).each do |i|
      x = Integer(color.slice(i*2,2), 16).to_s(10).to_i
      x = Integer([[0,x+(x*lum)].max,255].min.round.to_s, 10).to_s(16)
      rgb << ('00'+x)[x.length..-1]
    end
    return rgb
  end