Green was the version paired with Red in Pokémon’s original Japanese release.
Pocket Monsters Red and Green launched in Japan in February 1996. Outside Japan, the pair was released as Pokémon Red and Blue. The international naming difference is one reason anniversary discussions sometimes list the first games as Red and Blue even though the original Japanese release was Red and Green.
Japan later received Pokémon Blue as a special version with updated artwork, dialogue, and other changes. The overseas Red and Blue releases were based technically on the Japanese Blue version while retaining the Pokémon distribution associated with the original Red and Green pair.
The colors identify different game versions rather than completely different adventures. Each version contains Pokémon unavailable in the other, encouraging players to connect their Game Boy systems with a Link Cable to trade and complete the Pokédex. Pokémon Yellow followed later as an enhanced version.