It's not a silver Chocobo, it's a White Chocobo. You can get him to join your party if you give the White Chocobo in Mideel some Mimett Greens and tickle him behind the ear, but have your Materia inventory full up so you can't hold any more Materia.
Because you can't hold the Contain Materia the White Chocobo will join your party instead.
(SPOILER)That's a totally blatant lie (obviously). The big clue being the near-impossible way to fulfil the game's requirements.
