Kiaser Hi, David will give you the definitive answer, but my understanding is, it was dependent on the theatre of operation,
The Tunisian campaign used Desert camo colours, whereas most other areas of service it was dark green, I wait David showing me to be wrong (again) but I do not think the German armed forces used grey fire extinguishers.
My African Tanks extinguishers are painted Desert Sand and my Barbarossa Tanks extinguishers are green.
I suppose you know that "PEDDINGHAUS" sell fire extinguisher decals, a bit of a pain to apply but really improves the look.
A big problem with colour in German equipment is simply that finding original colour photographs is very difficult so that a lot of the information is based on documentation,
And that is why, fortunately we have the likes of David to come riding to the rescue

shaun