It seems that the standard color form the manufacturer was a medium gray while those ordered for the military came in the required color of the time, Dunkelgrau or Dunkelgelb. However, these got overpainted at the unit level so the label isn't clearly seen when this happens. The manufacturer also made them in red but this should be limited to interiors although some have state (seems opinion and not fact) that they were used inside halftracks as well. Note that there are various types of extinguishers, based on the vehicle they were mounted.
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The Pz.Kpfw. I had a thinner extinguisher and this Japanese modeler did an EXCELLENT job of research along with artwork for them. Click image to read article in Japanese.
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