The following is the effects of greens when fed to your chocobos for maturing.
Memit Greens = Speed
Gizzard Greens = Speed & Stamina
Karakka Greens = Intelligence
Tantal Greens = Speed, Stamina & Intelligence
Pasana Greens = Intelligence
Curie Greens = Speed & Stamina
Reagan Greens = All stats
Sylkis Greens = All stats
Each of the greens has its advantages and disadvantages. For example, Tantal boosts Speed, Stamina & Intelligence, but Memit boosts Speed far more than Tantal.