def similarity( color1, color2, ignore_lightness: false )
      l1, a1, b1 = *rgb2lab(color1.to_rgb)
      l2, a2, b2 = *rgb2lab(color2.to_rgb)
      Math.sqrt( (ignore_lightness ? 0 : (l1-l2)**2) + (a1-a2)**2 + (b1-b2)**2 )
    end
def similarity( color1, color2, ignore_lightness: false )
      l1, a1, b1 = *rgb2lab(color1.to_rgb)
      l2, a2, b2 = *rgb2lab(color2.to_rgb)
      Math.sqrt( (ignore_lightness ? 0 : (l1-l2)**2) + (a1-a2)**2 + (b1-b2)**2 )
    end
使用 CIE76 公式计算颜色的相似程度
 参考:
 - [CIELAB](http://en.wikipedia.org/wiki/CIELAB)
 - [Color Difference](http://en.wikipedia.org/wiki/Color_difference)