Meine Herren !
An excellent source of WW I colours are Autochrome photographies.
Autochrome method delivers real colour photos (slides they are) and subject information tells that it renders colours very(!) natural .
The Verlag Militaria Stefan Rest in Austria used some in their top-volume on the French Army and the banker Kahn/France, who
intended to photograph the whole world in colour, did WW 1 autochromes, too (a book and DVD by BBC TV is available via Amazon).
Hacking-in the Name Paul Castelnau I came across a Website called Luminus Lint and there were autochromes of another war
photographer. I found photos of airplanes, too !
So I think searching the WWW for autochromes is rewarding !
It is even moving sometimes, because colour breathes life into bygone days which we even think of in b/w !
So -take a look on the Autochrome side !
Kind regards, Gunther