Some old fire extinguishers are still serviceable. I've seen quite a few of them in my time. Need to ask a competent person to check them out.

Actually despite the recommendations of BS EN 3 regarding fire extinguishers being red it's not a legal requirement and you can have them purple with yellow spots if you like provided all your staff know how to identify them, what they are for, how to use them and if challenged or if an incident occurs you can convince the FRS and/or the Courts that this was not a factor in any harm suffered by anyone!

This argument might be difficult to make if half your foam extinguishers were red and half were cream. (I assume this is what you mean by yellow?) Painting the cream ones red is perfectly fine as long as they are serviced and checked (if you can get anyone to do it!) but don't mix the two on site.

However Chris's point about cost effectiveness is well made. Many fire extinguisher companies don't go down the "major strip down and service" route any more and will simply offer you a cheaper replacement extinguisher when your current ones are 5 or 10 years old. Thus it's unlikely that you will find anyone who will service and refurbish these old extinguishers for a lower cost than simply buying new ones. Sad really.

I've found this with CO2 extinguishers as well. despite the fact that with the correct pressure test regime these could last many years they simply tend to get swapped out for new ones when they are ten years old.