Costume Closet

The costume closet is said to be quite a rare anomaly that allows you to change Mario’s current outfit.

Manifestation
It’s unknown what conditions the costume closet spawns under, although it appears that you can’t have any unintended playable characters, otherwise it seems impossible for it to manifest. When it does manifest in your copy it will usually appear in the anomalous rec room, although if the rec room doesn't manifest in your copy it will likely spawn in the room with the castle’s secret slide. When it appears you may enter it, doing so will bring you to a menu where you can change Mario’s costume. The menu itself has never been described in much more detail then a menu with a Mario picture with the currently selected costume and a light brown background, along with arrows to select which costume you want to wear.

Mario outfit list
Below are colors that have been documented, although if you do find a new one feel free to report it here.

Unlockable Mario outfit
Players who have seen this anomaly have reported that sometimes one to three of the costumes need to be unlocked through various ways like collecting a certain number of coins, defeating a certain amount of enemies and beating a certain stage.

Theorizing
It’s widely agreed that the costume closet was cut along with the rec room, although many people are confused by it appearing in the secret slide room.

Some people think that the reason Luigi's costume has purple overalls to make sure that if the player is playing multiplayer mode that the two can be distinguished when playing the mode.