Inserting the Action Replay
The Action Replay for Game Boy Color is a cartridge similar to the ones used for games. The game that you want to cheat on is inserted into a slot on the Action Replay. The Action Replay itself is placed in the game slot on the Game Boy Color.
Using Action Replay
Turning on the Game Boy Color will automatically cause the Action Replay code list for the inserted game to come up. The Action Replay cartridge comes pre-loaded with cheats for most Game Boy games. These cheats can be selected from the code list. A check mark will appear next to the cheat name to indicate it will be activated when the game starts. To start the game with the cheats, press the "Start" button and select "Play Game" from the menu that comes up.
Adding Codes
It is possible to add additional cheats to the Action Replay by entering correct codes. This is done through the game's code list. Select the "Add New Cheat" from the top of the code list. Enter a description of the cheat; this will be what will appears on the code list for future reference. Enter the code. The cheat will be saved to the Action Replay and appear in the cheat list when you play the game in the future.
Warnings
Using the Action Replay on Game Boy Color games will sometimes corrupt the saved files on the game. You can potentially lose entire saved games to this. Some cheats can cause some parts of the games to become unbeatable because they interfere with events that should happen in the game. For example, activating an invincibility cheat could potentially prevent you from losing plot fights that you are supposed to lose in order to advance the storyline.