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 the player 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 Koopa without a Shell goes inside one, it becomes a flashing Shell and chases the player. When Yoshi eats a Yellow Shell, he gains an ability to create sand clouds on either side of himself to defeat nearby grounded enemies for a limited time, at the end of which he swallows the shell. A Yellow Yoshi can gain this ability by eating a shell of any color. Yoshis can also spit the shell back out, in which case it behaves like a Green Shell, flying out at high speed and defeating enemies in its way. If the player attempts to launch the shell at a blue Koopa without a Shell, it stops the shell and kicks it back at the player. If a green or red Koopa without a Shell comes in contact with the Yellow Shell (or kicks it right-side up if upside down), it then jumps in it and becomes that colored Koopa.

After the player completes the Special Zone, or finds 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 standalone 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 random items 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 they touch except for Bowser. When they collide with Bowser, they spin in place until Bowser walks through them. 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 capsules. Like other shells, some can be trapped in ice, which require two clearings to be able to use them.

Nintendo Kōshiki Guidebook – Yoshi's Egg
黄 ちゃんこ鍋の味がする変わり種のこうら. 関取エキスを多置に含んでいるので、これを食べるとあら不思議、体重が増える. つまり黄色ヨッシーの技が使えるのだ.

Yellow ''A strange kind of shell that tastes like chankonabe. It contains a large amount of sekitori extract, so if you eat this wonder, you'll gain weight. That is, you can use the yellow-colored Yoshi's skill.''

Trivia

 * In Super Mario Bros. Special (NEC PC-8801), Green Koopa Troopas have yellow shells due to palette limitations.
 * According to the Japanese Shogakukan guide for the game Yoshi, Yellow Shells from Super Mario World taste like and contain a lot of  extract, promoting weight gain.