code: def check_color(cls, raw_image):
          
          # sum(img.convert("L").getextrema()) in (0, 2)
          extrema = raw_image.convert("L").getextrema()
          if extrema == (255, 255): # all white
              raise cls.MonoImageException
docs: Just check if raw_image is completely white.
          http://stackoverflow.com/questions/14041562/python-pil-detect-if-an-image-is-completely-black-or-white