Colour tells you what is in a cylinder. Class tells you what it is for. The two are related but they are not the same question, and a duty holder who works from colour alone will eventually put a foam unit in front of a fryer. The fire classes are a British and European classification of fuels, and every extinguisher carries the classes it is rated against printed on its own label.
The classes, in the order they appear on a label
Class A is carbonaceous solids: paper, wood, textiles, most rubbish. Class B is flammable liquids: solvents, petrol, paint, spirits. Class C is flammable gases, which portable units are rarely provided for because the correct action is to isolate the supply. Class D is combustible metals. Class F is cooking oils and fats. Electrical risk is not a class at all, it is a marking, because electricity is an ignition source rather than a fuel.
Which agent covers which class
Water covers A. Foam covers A and B. Dry powder covers A, B and C and is marked safe on live electrical equipment. Carbon dioxide covers B and is marked for electrical risk. Wet chemical covers F and usually A. Nothing covers every class, which is why a building with mixed risks holds more than one type rather than a single universal unit.
Matching the provision to the room
An office with paper and computers is a water or foam provision with carbon dioxide near the server equipment. A workshop with solvents is a foam or powder provision. A commercial kitchen is a wet chemical provision with a fire blanket, whatever else the building carries elsewhere. The risk assessment is what decides this, and it has to name the rooms.
The classes a portable unit should not be asked to fix
A gas fire that is still fed is not an extinguisher problem. A metal fire treated with water becomes worse rather than better. A deep fat fryer treated with water throws burning oil across the room. These are the cases where the honest instruction is to raise the alarm and leave, and a fire risk assessment that pretends otherwise is doing a building harm.
Questions people ask about different fire extinguishers and their uses
Is there a single extinguisher that does everything?
No. Dry powder comes closest on paper because it is rated for three classes and is safe near electrics, but it is refused in most occupied interiors because of what it does to visibility and to equipment when it discharges.
What does the electrical marking mean?
It records that the agent has been tested as safe to use in the presence of live electrical equipment at a stated voltage and distance. It is not a licence to fight a fire in an energised panel; the first action is still isolation where that can be done safely.
Why do extinguisher labels show pictograms?
Because the classes have to be readable by somebody who has not been on a course. The pictograms carry the same information as the class letters and are the part a person actually uses when choosing under pressure.