docs: def rgb(red, green, blue):
      """
      Calculate the palette index of a color in the 6x6x6 color cube.
  
      The red, green and blue arguments may range from 0 to 5.
      """
      for value in (red, green, blue):
          if value not in range(6):
              raise ColorError('Value must be within 0-5, was {}.'.format(value))
      return 16 + (red * 36) + (green * 6) + blue