"The Battle of the Bulge" was a staggeringly bad film (and I speak from a position of some authority, as a member of a nation which may well lead the world in the creation of truly diabolical films).  

In September 1989, the BBC had the brilliant notion of prefacing its news reports with mock-up news reports on the early weks of WWII, as they might have appeared in September, 1939, if there had been television news reports back then.   The one war theatre of intense activity in September '39 was Poland, of course, and there isn't a huge amount of footage of the campaign, not surprisingly.   The 1989 BBC did, however, have access to reams of contemporary film of British and French troops "in action" (on manoeuvres, presumably), so they  used footage of the British to fill in where the commentary was referring to the Nazis (thanks, Beeb) and footage of the French to depict the Poles.

Conversion by paint can work in some contexts and I think that the Napoleonic era is one of them (within reason, of course).   You really can re-colour 1812 British/KGL to get 1812 US and 1812 Portuguese, because all those uniforms were made in the same English mills.   I suspect quite a lot of continental Europe either had to accept French manufacture, or, at least, was expected to conform with French patterns.   It was all part of the mutual blockades being conducted by GB and France.

As AntoniaTiger quite rightly says, though, that is not going to work for the twentieth century.   You can't stick 1967 Israeli into Photoshop and come out with 1956 Algiers, or 1943 Kursk.