Hi there,
It is my understanding that, when the extinguisher was inside the vehicle, if you paint it flat/matt green or the hull color, you will be doing an accurate painting job.

Cheers,

Pablo

On the outside it makes no sense to have something that sticks out like a red extinguisher. Red may have been used pre-war and even in the early stages. For example, the white crosses on German tanks were subdued during the Polish campaign as they were a big bulls eye for the Polish gunners. Then why bother using dark grey or cammo paint on the outside? The tools were probably over sprayed also to match the tank color. Bare metal would reflect light and give away the tank's position so they too were probably painted. It's hard to tell from the period photographs.
As for the inside extinguisher color, red may have been used too since it may be easier to spot in an emergency. What you may find in a reference photo may or may not be correct, specially true in 'restored' museum vehicles.