Pokémon Blue replaced Green in the international releases of the original Pokémon pair, creating Pokémon Red and Blue outside Japan. Japan first received Pokémon Red and Green on February 27, 1996.
Japan later received a revised Pokémon Blue, initially as a limited mail-order edition and later as a general retail release. Its updated artwork, dialogue, and technical foundation were used for the international versions.
The international Red and Blue games still preserved the idea of version-exclusive Pokémon, so trading remained important. However, they were not simply the Japanese cartridges with translated text; they were based on the updated Japanese Blue version.
Pokémon Yellow followed internationally as a special edition centered on Pikachu and influenced by the anime. It did not replace Green in the original pair, which is why “Red and Blue” is the familiar international launch name.