code: def color_string(self, x):
          
          diff_str = ""
          color = "black"
  
          if len(x) == 2 and self.compare_file is not None:
              difference = x[0] - x[1]
              if difference:
                  color, sign = ('green', '-') if difference < 0 else ('red', '+')
                  diff_str = '{}{}'.format(sign, self.format_measure(difference))
          return [self.format_measure(x[0]), [diff_str, color]]
docs: Return a string formatted delta for the values in x.
  
          Args:
              x: 2-item list of integers (representing number of calls) or
                 2-item list of floats (representing seconds of runtime).
  
          Returns:
              A list with [formatted x[0], [color, formatted delta]], where
              color reflects whether x[1] is lower, greater, or the same as
              x[0].