Some PokéStops are yellow because they have a Golden Lure Module placed on them. This Golden Lure Module turns the PokéStop gold or yellow, and it functions similarly to a normal lure but only attracts a specific Pokémon species called Gimmighoul. Spinning a yellow or Golden PokéStop can give players Gimmighoul Coins, which are needed to evolve Gimmighoul into Gholdengo. These Golden Lure Modules are obtained by linking Pokémon GO to Pokémon Scarlet/Violet and sending postcards in the game. Yellow PokéStops can also sometimes appear during special events or Go Fest, where they drop more items and the Gimmighoul Coins. The yellow color indicates the special status of the PokéStop due to the Golden Lure Module or event effects.
Additionally, there are other instances of yellow PokéStops representing Team GO Rocket takeover, but the more common reason currently in Pokémon GO for yellow/gold PokéStops is the Golden Lure Module attracting Gimmighoul.
