docs: def add_colors(self, system, colors):
          """Add color definition to a given color system.
  
          You may add to already existing color system. Previously existing color
          definitions of the same (normalized) name will be overwritten,
          regardless of the color system.
  
          Args:
            system (string): The color system the colors should be added to
              (e.g. ``"en"``).
            color_definitions (iterable of tuples): Color name / sRGB value pairs
              (e.g.  ``[("white", "ffffff"), ("red", "ff0000")]``)
  
          Examples:
            >>> color_definitions = {"greenish": "336633", "blueish": "334466"}
            >>> tint_registry = TintRegistry()
            >>> tint_registry.add_colors("vague", color_definitions.iteritems())
  
          """
  
          if system not in self._colors_by_system_hex:
              self._colors_by_system_hex[system] = {}
              self._colors_by_system_lab[system] = []
  
          for color_name, hex_code in colors:
              hex_code = hex_code.lower().strip().strip("#")
              color_name = color_name.lower().strip()
              if not isinstance(color_name, unicode):
                  color_name = unicode(color_name, "utf-8")
  
              self._colors_by_system_hex[system][hex_code] = color_name
              self._colors_by_system_lab[system].append((_hex_to_lab(hex_code), color_name))
              self._hex_by_color[_normalize(color_name)] = hex_code