From the Super Mario Wiki
A Pepper is a type of spicy fruit that can either be green, red, or blue. They are the most hated food of the Yoshis. However, Black Yoshis and White Yoshis seem to like them in Yoshi's Story. If a regular Yoshi eats one, they will lose one petal from their Smile Meter.
Red and Green Peppers appear in Mario & Luigi: Superstar Saga, and both of these, plus a new Blue Pepper appear in Mario & Luigi: Partners in Time. In the Mario & Luigi series, they serve as stat boosters; Green Peppers raise defense and reduce weight, Red Peppers raise attack and weight, and Blue Peppers raise speed. In Mario & Luigi: Superstar Saga, they only affect one character, but in Mario & Luigi: Partners in Time, they affect all characters currently fighting. Also in the latter game, the gravity changes are removed.
Peppers also appears with the other fruit in Super Mario Sunshine at Delfino Plaza. If a Yoshi eats one, it turns purple, but a Yoshi never requires one to hatch out of its egg. It is also used to defeat King Boo at the casino of Hotel Delfino. By throwing a pepper at his tongue, King Boo is stunned, allowing Mario to damage him with another fruit.
A different type of pepper called the Dash Pepper appears in the game Super Mario Galaxy 2 which, when eaten, allows Yoshi to run at an extraordinary speed. Peppers also made appearances in The Super Mario Bros. Super Show!, but only in one episode.
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