docs: def find_nearest_color_hexstr(hexdigits, color_table=None, method='euclid'):
      ''' Given a three or six-character hex digit string, return the nearest
          color index.
  
          Arguments:
              hexdigits:  a three/6 digit hex string, e.g. 'b0b', '123456'
  
          Returns:
              int, None: index, or None on error.
      '''
      triplet = []
      try:
          if len(hexdigits) == 3:
              for digit in hexdigits:
                  digit = int(digit, 16)
                  triplet.append((digit * 16) + digit)
          elif len(hexdigits) == 6:
              triplet.extend(int(hexdigits[i:i+2], 16) for i in (0, 2, 4))
          else:
              raise ValueError('wrong length: %r' % hexdigits)
      except ValueError:
          return None
  
      return find_nearest_color_index(*triplet,
                                      color_table=color_table,
                                      method=method)