You probably don't think much about fire extinguishers beyond making sure they're tested and working properly. Maybe you have noticed the colours on the labels and wondered what they mean. Designated purpose and cultural preferences are the 2 biggest determinants behind the colours of fire extinguishers.
Fire extinguishers are metal canisters containing materials that will, when released, put out fires.
The first fire extinguisher used a blast of gunpowder to send water exploding over a fire. Colors originally represented contents--red for water, blue for powder, yellow for foam, black for carbon dioxide.
Some substances can make fires worse--water on electrical or grease fires, for example--so chemicals have been developed to fight different types of fires. The United States organises fires into 5 types. Some countries use 6 types.
Color coding today represents the type of fire for which an extinguisher is designed and is more common in Europe and the United Kingdom than the United States.
As more chemicals were developed for fighting fires, labelling became more complicated and new colours were added. Multi-use extinguishers had to display bands of colours.
Extinguisher colour codes have evolved differently in many countries. Although colours are still used in some applications, extinguisher contents are now universally identified by labels that display letters or pictographs that represent the types of fires for which they are effective.