The following are known Action Replay Codes for Return of the Ninja 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 Health
910f5cc8

Invisible!
910632cb

Have Power Flame
910100cb

Ninjutsi Spells Modifier
91xx5fc8

Fire Ball
01

Ice Shards
02

Lighting
03

Tornado
04

Stones
05

Have Iron Claw
910160c8

Have Tabi
910161c8

Have Kabuto
910163c8

Have Mantle
910164c8

Have Mizugumo
910165c8

Super Weapon
91805aca

Super Jump
910162c8