docs: def find_nearest_color_index(r, g, b, color_table=None, method='euclid'):
      ''' Given three integers representing R, G, and B,
          return the nearest color index.
  
          Arguments:
              r:    int - of range 0…255
              g:    int - of range 0…255
              b:    int - of range 0…255
  
          Returns:
              int, None: index, or None on error.
      '''
      shortest_distance = 257*257*3  # max eucl. distance from #000000 to #ffffff
      index = 0                      # default to black
      if not color_table:
          if not color_table8:
              build_color_tables()
          color_table = color_table8
  
      for i, values in enumerate(color_table):
          rd = r - values[0]
          gd = g - values[1]
          bd = b - values[2]
  
          this_distance = (rd * rd) + (gd * gd) + (bd * bd)
  
          if this_distance < shortest_distance:  # closer
              index = i
              shortest_distance = this_distance
  
      return index