code: def _darkest(self):
          
          
          
          rgb, n = (1.0, 1.0, 1.0), 3.0
          for r,g,b in self:
              if r+g+b < n:
                  rgb, n = (r,g,b), r+g+b
          
          return rgb
docs: Returns the darkest swatch.
          
          Knowing the contract between a light and a dark swatch
          can help us decide how to display readable typography.