For Safety....
-------> .Red.. #2 which is Compressed Hydrogen.

Hydrogen (H2) is the lightest of all gases. Colorless, odorless, tasteless and nontoxic, hydrogen exists as a gas at atmospheric temperatures and pressures. In metal fabrication, hydrogen serves as a protective atmosphere in high-temperature operations such as stainless steel manufacturing; commonly mixed with argon for welding austenitic stainless. Hydrogen is used to enhance plasma welding and cutting operations.

H2 is used in many applications including a fuel source alternative to gasoline, diesel.