Gas cylinders have internationally recognised markings and colours on the body and shoulders of the cylinder, also written labels or collars on the neck.
In future the colour will only be on the shoulder of the cylinder – this will the recognised visual way to identify a cylinder hazard.
COLOUR indicates the hazard.
LABEL identifies the gas.
Cylinders also have a different thread on the valve connection for some gas types.
Gas suppliers will provide information on cylinder markings – e.g. below
A technical information sheet from the BCGA [British Compressed Gas Association]