Gas supplied under pressure in metal cylinders. The term includes pressurized gas (e.g. oxygen and nitrogen cylinders) and gases liquefied under pressure (e.g. liquid butane for use as a fuel). Colour conventions are used to identify the type of gas or, in some cases, the specific gas. The colour indicating the contents is that of the shoulder of the cylinder at the top. The convention is not international, and practice differs in different countries. In the UK, the convention is:

• Yellow for toxic or corrosive gases

• Red for flammable gases

• Light blue for oxidizing gases

• Maroon for acetylene

• Dark green for argon

• Grey for carbon dioxide

• Brown for helium

• Blue for nitrous oxide

• Black for nitrogen

• White for oxygen

https://www.boconline.co.uk/en/health-and-safety/gas-safety/identifying-gas-cylinders/industrial-gas-cylinder-colours/index.html Information on colour coding of gas containers