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Musuda explains why Pokemon Grey never happened after Pokemon Black/White
Many people thought Pokemon Co. would release a Pokemon Grey title as a follow-up to Pokemon Black/White, but that never came to be. Now we've find out why that is, thanks to Game Informer's chat with Junichi Masuda...
Somewhat related – they did talk about why Pokémon Grey wasn't a thing after Black and White. Masuda said thematically, the game is about the opposition of good and evil, black and white, yin and yang, etc., and to have a Pokémon Grey would have been in opposition of that theme.
We also have the full statement straight from Masuda...
A lot of people were expecting Pokemon Gray at the time, but the concept of Black and White was kind of these opposing forces – a yin-yang kind of thing. If we went with Gray, it would have moved away from that concept so we decided to keep the titles there.