The Luigi Cap color was taken from the Shellcreeper's palette to distinguish Mario from Luigi. The cap was made green with a green "L" emblem, but it was white on Super Mario Bros. and Super Mario Bros.: The Lost Levels.
In Super Mario Bros. 2, Super Mario Bros. 3, and Super Mario World, the tip of the cap had been changed to black instead of its usual green. This happened for the Mario Cap as well. However, after this game, the Luigi Cap reverted to having a green tip.
In Super Mario 64 DS, Luigi's Cap was used to shape-shift into Luigi if the player was either Mario, Wario, or Yoshi. In the same game, Luigi is affected in the same way as Mario and Wario if his cap is lost. Luigi's defense and attack sharply decline while his cap is absent from his head. If the player gets the cap back, Luigi's attack and defense revert to normal. Until Luigi is unlocked, it only appears in Big Boo's Haunt, on any star except the first one. If the player obtains a Power Flower while wearing the cap, they will turn into Vanish Luigi. If he loses his cap and gains a star from another level, Luigi can simply talk to Toad (who will have found and be wearing Luigi's Cap).
In the first three games of the Mario & Luigi series, the "L" on the Luigi Cap is black. The Mario Cap was changed in the same way. From Mario & Luigi: Dream Team onward, they are the usual green and red. In the first three games of the Super Smash Bros. series, the Luigi Cap looks more realistic. The Mario Cap was changed in the same way. From Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U and onward, they look more cartoonish as usual.
In Super Mario 3D Land and Super Mario 3D World, Luigi, like his brother, loses his cap in his Small form. However, in the former game, once the player has 1110 lives, his regular form loses his cap instead.
The Luigi Cap can be obtained for Mario to wear in Super Mario Odyssey. It can either be purchased from a Crazy Cap store or obtained through scanning a Luigi amiibo (the latter also unlocks Luigi's costume).
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