Colour coding is used primarily to identify the hazard associated with the contents of a cylinder. Its importance cannot be over-emphasised, and this has led to the International Marine Contractors Association (IMCA) publishing guidance on ‘Marking and Colour Coding of Gas Cylinders, Quads and Banks for Diving Applications'.
The guidance (IMCA D 043) is applicable internationally, but, national
regulations/standards will normally prevail.
In addition to commenting on this, the guidance has a useful chart showing commonly accepted colour coding for helium, medical oxygen, oxygen and helium mixtures, nitrogen, oxygen/helium/nitrogen mixtures, air (breathing), oxygen/nitrogen mixtures, carbon dioxide, and calibration gases.
It also illustrates gas mixtures for medical or inhalation purposes; and shows typical examples of quads and banks (in addition to carrying written descriptions) and also of a cylinder labelling system.