code: def get_unused_color(self):
          
          if not self.unused_colors:
              # If we somehow run out of colors, reset the color matcher.
              self.reset()
          used_colors = [c for c in self.xlwt_colors if c not in self.unused_colors]
          result_color = max(self.unused_colors,
                             key=lambda c: min(self.color_distance(c, c2)
                                               for c2 in used_colors))
          result_index = self.xlwt_colors.index(result_color)
          self.unused_colors.discard(result_color)
          return result_index
docs: Returns an xlwt color index that has not been previously returned by
          this instance.  Attempts to maximize the distance between the color and
          all previously used colors.