Observation can sometimes be done, second hand, so to speak.
A friend of mine collects dvds of WW2 colour footage taken by cameramen from the various combatants.
One of the more curious sets he has is of footage taken by a German officer, post 1941, using Kodachrome colour film, of German troops and equipment in action.
Kodachrome could only be processed in one location in the world at that time, and that was in the USA.
This officer managed to get a supply of the stuff, but also to then send it to the USA for processing, probably through a contact in Spain, Sweden or Switzerland, and then have it sent back to him once processed during the war.
It is not too much to theorize that Kodak knew what they were dealing with, and that they provided copies to the US War Department for analysis.
The guy could have used Agfacolor, which was an easier to process film, but instead chose Kodachrome due to its superior, at the time, colour qualities.