From the Super Mario Wiki
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Yellow Shells are yellow Koopa Troopa shells found originally in Super Mario World. When a Yoshi lands with a Yellow Shell in his mouth, a shockwave will surround him. This will cause the enemies to flip over and it will defeat all nearby grounded enemies. Yoshis can also spit it back out, in which case it will behave like a Green Shell, flying out at high speed and knocking over all enemies in its way.
Like other Koopa Troopa shells, the Yellow Shell will be launched when the Koopa Troopa wearing it is jumped on. It can also be held and thrown. If the player attempts to launch the shell back at a blue Beach Koopa, then it will grab and throw it back at the player.
In the Glitz Pit (Chapter 3) of Paper Mario: The Thousand-Year Door, the KP Koopas are the 18th ranked fighters. They have the same stats as a regular Koopa Troopa but they are yellow instead of red. KP Pete is the leader of the team, and the first character of the chapter to introduce himself to Mario.