In the European Union gas cylinders are beginning to be colour coded according to EN 1098-3. The "shoulder" is the top of the cylinder close to the pillar valve. For mixed gases, the colours can be either bands or "quarters".

* Air has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) "quartered" shoulders.
From above link.