The following are known Action Replay Codes for Commander Keen on Nintendo Game Boy / Color (GB / GBC). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

Infinite Lives
916334c9

Infinite Continues
91633dc9

Max Score
917f35c9
919636c9
919837c9

Have All Keys
91ff95c9

Have All 3 Crystals
91ff3ac9
91ff3bc9