Complex sentence: Although not part of Nintendo at the time, Iwata assisted in the development of "Pokémon Gold" and "Silver", which were released for the Game Boy Color in November 1999, by creating a set of compression tools utilized for graphics in the games.
Split into simpler sentences: Although not part of Nintendo at the time, Iwata assisted in the development of "Pokémon Gold" and "Silver", which were released for the Game Boy Color in November 1999. He created a set of compression tools utilized for graphics in the games.