Glitch dimension

The glitch dimension (as it is often referred to by fans) is a glitch in. Unlike some other glitches, this glitch is merely for altering the visuals of the game, and will not negatively affect or delete the saved file once it is executed.

Results
Several different things can happen upon activating this glitch, one being that the game freezes upon activating it. The other is that the screen turns white with a small pink rectangle in the top right-hand corner and the game restarts with different colors. When activated, it alters the colors of everything from the area the undefined walks around in to the wild and battles. Also, it slows the processing speed when trying to view the player's party Pokémon when not in battle. Pitch black places like Dark Cave will be illuminated without the use of the move. The Poké in Pokégear changes to ど◀ when the player enters their Pokémon selection. All fade-in and fade-out transitions are disabled.

Interestingly, the on the title screen of Gold Version changes from a silhouette to a fully colored Pokémon.

Coin Case method
Either talking to the in Vermilion City, the  in the Goldenrod Department Store or viewing the Pokédex entry of Machop, Machoke or  and using the Coin Case immediately afterwards, will cause the game to enter the glitch dimension.

Ditto method
Trading a Pokémon with the glitch move onto a Gold, Silver or, then using -- in battle will cause the game to enter the glitch dimension.

Other methods
Move hex:FE (a move with a blank name, thus is referred to here as its hexadecimal identifier) has also been found to cause glitch dimension, but this has not been confirmed outside of Crystal on emulator, as in this video.

Explanation
The glitch causes the game to reset unexpectedly. When this happens it repeats the process used at startup to determine what type of Game Boy it is running on. This process relies on the fact that different models leave different information in memory at startup. Since the game was already running, that information is no longer present, so the process fails and believes it is running on a classic Game Boy. Because of this, the game is trying to ensure backward compatibility by disabling all changes in color palettes, since the original Game Boy did not support them.

As the game thinks that it is running on the first model of Game Boy, certain features will be disabled. For example, talking to the girl in the Goldenrod Department Store who enables Mystery Gift will tell the player that Mystery Gift requires a Game Boy Color. completely abandoned any backward compatibility with the original Game Boy, so performing this glitch on Crystal will cause the game to show an error message, explaining that a Game Boy Color is necessary to play the game.

In Generation I
An effect similar to Glitch dimension can occur in, when running the game on Game Boy Color or in SGB mode. In the glitch happens most commonly as a side-effect of corrupting the memory, or after performing the ZZAZZ glitch. In the effect can be rarely activated by encountering certain glitch Pokémon, like MissingNo. or 4 4.