def hue_similarity( color1, color2 )
      deg1   = color1.to_hsl.h
      deg2   = color2.to_hsl.h
      deg1  += 1 if deg1 < 0
      deg2  += 1 if deg2 < 0
      delta  = (deg1 - deg2).abs
      delta  = 1 - delta if delta > 0.5
      delta * 360
    end
def hue_similarity( color1, color2 )
      deg1   = color1.to_hsl.h
      deg2   = color2.to_hsl.h
      deg1  += 1 if deg1 < 0
      deg2  += 1 if deg2 < 0
      delta  = (deg1 - deg2).abs
      delta  = 1 - delta if delta > 0.5
      delta * 360
    end
Public: 判断两种颜色的色系相似程度
         在色盘上越接近，返回值越小

 Returns 色相距离，0-180之间的角度值