Yellow Shell

Yellow Shells are yellow Koopa Troopa shells that are worn by Koopa Troopas.

Super Mario World
Yellow Shells first appear in Super Mario World, one of the few games where they are actually worn by Koopa Troopas. When jumping on the yellow-shelled Koopa Troopas, they will lose their shell and a coin pops out from it, standalone yellow shells also appear in some levels. Like other Koopa Troopa shells, the Yellow Shells 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 Beach Koopa goes inside of one, it will become a flashing shell and chase after the player. When Yoshi lands with a Yellow Shell in his mouth, he can temporarily use sand clouds that surround him while he has the shell in his mouth or before he is about to swallow it. This will cause the enemies to flip over and it will defeat all nearby grounded enemies. Yoshis can also spit it back out (except for the red ones, due to it turning into fireballs after being spitted out), in which case it will behave like a Green Shell, flying out at high speed and knocking over all enemies in its way. Yellow Yoshi can use this ability from any shell color. If the player attempts to launch the shell back at a blue Beach Koopa, then it will grab and kick it back at the player.

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, including with the yellow variants become Mask Koopas, with their respective shells becoming yellow capped masks that vaguely resemble Mario.

Yoshi's Safari
In Yoshi's Safari, when yellow-shelled Koopa Paratroopas are defeated, they retreat in their shell and disappear.

Mario's Time Machine
Yellow Shells also appear in Mario's Time Machine for the NES, unlike in Super Mario World, jumping on one does not release the Koopa out of its shell and instead retreats inside of 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 temporary, rather than damaging him.

Paper Mario
In Paper Mario, Yellow Ninjakoopa uses a Yellow Shell when he is stacked with the other Koopa Bros. to use whirling fortress or when he shell tosses at Mario separately.

Paper Mario: The Thousand Year Door
In the Glitz Pit (Chapter 3) of Paper Mario: The Thousand-Year Door, the KP Koopas which squad consists of a KP Paratroopa and two KP Koopas, including the team's leader, King Pete uses their Yellow Shells to attack Mario and his partners during battle. They are simply Koopa Troopas with their shells dyed yellow (refereed to as Gold in the French and Italian releases).

Mario Party Advance
Yellow Shells appear in the mini-game of Mario Party Advance, Shell Stack. They appear after a few Green Shells are launched, once the Pipe Cannon turns yellow, it will shoot out a Yellow Shell and are lightly faster than the Green Shells.

Puzzle & Dragons: Super Mario Bros. Edition
In Puzzle & Dragons: Super Mario Bros. Edition, Yellow Koopa Troopas, Koopa Paratroopas and their big relatives use their shells to attack the player.

Dr. Mario World
Yellow Shells reappear in Dr. Mario World. It is one of the colored shells that appears as a stage item in the game, it clears a row when it has been matched with the corresponding Megavitamins.

Trivia

 * In Super Mario Bros. Special (NEC PC-8801), Green Koopa Troopas have yellow shells due to palette limitations.
 * Due to palette limitations, Super Mario World has Ludwig von Koopa using the palette intended for the yellow Koopa Troopas, as a result, his shell also becomes yellow.

Gelber Panzer