Yellow Shell

Yellow Shells are one of the rare colored variants of the Koopa shell, first appearing in Super Mario World.

Super Mario World
Super Mario World is one of the few games where Yellow Shells are actually worn by Koopa Troopas. Upon jumping on a yellow-shelled Koopa Troopa, it loses its shell and a coin pops out from it. Standalone yellow shells also appear in some levels. Like other Koopa Troopa shells, the Yellow Shell can be launched when the Koopa Troopa wearing it is jumped on and removed from its shell. It can also be held and thrown. If a yellow Beach Koopa goes inside of one, it becomes a flashing shell and chases after the player. When Yoshi lands with a Yellow Shell in his mouth, he can create sand clouds on either side of himself while he has the shell in his mouth or before he is about to swallow it, which defeats nearby grounded enemies. (Yellow Yoshi can use this ability from any shell color.) Yoshis can also spit the shell 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. If the player attempts to launch the shell back at a blue Beach Koopa, it will grab and kick it back at the player. If a green or red Beach Koopa comes in contact with the Yellow Shell (or kicks it right-side up if the shell is upside down) it will then jump in it, and become that colored Koopa.

After the player completes the Special Zone, or find all exits in Super Mario World: Super Mario Advance 2, like with all Koopa Troopas and Koopa Paratroopas, the yellow variants become Mask Koopas, with their respective shells becoming yellow-capped masks that vaguely resemble Mario. Yellow Mask Koopas also replace the regular green Koopa Troopas (not including green Climbing Koopas), while the yellow masks replace the stand-alone Green Shells.

Mario's Time Machine
Yellow Shells also appear in Mario's Time Machine for the NES, replacing the common green-shelled Koopa Troopas. Unlike in Super Mario World, jumping on one does not release the Koopa out of its shell and instead causes it to retreat to their shells, and they can only be kicked to launch, knocking over all enemies in its way.

Getting hit by the shell simply stuns Mario temporarily, rather than damaging him.

Mario Bros.
Yellow Shells (simply referred to as Koopa Shells ) appear in the Game Boy Advance version of Mario Bros. included with the Super Mario Advance series and Mario & Luigi: Superstar Saga. They are a random item a player can find from within a garbage can. Once they are thrown, they slide across the ground, damaging any player or defeating any enemy it touches. They can also slide into the bottom Warp Pipes and will launch from the other Warp Pipe above. Standalone Yellow Shells are sometimes randomly launched from the Warp Pipes above.

Mario Party Advance
Yellow Shells make their Mario Party debut in the Mario Party Advance minigame Shell Stack. They appear after a few Green Shells have been launched; once the small Pipe Cannons turn yellow, one of them from left or right shoots out a Yellow Shell, which is slightly faster than the Green Shell.

Dr. Mario World
Yellow Shells reappear in Dr. Mario World as one of the colored shells that appears as a stage item in the game. They clear a row when they are matched with the corresponding Megavitamins.

Trivia

 * In Super Mario Bros. Special (NEC PC-8801), Green Koopa Troopas have yellow shells due to palette limitations.