Hi,
I agree with Stephen about dear old Heinz Nowarra. For its time,
Eisernes Kreuz und Balkenkreuz was a terrific book, and I still treasure my dog-eared, well-worn copy. Yes, Heinz made more than a few mistakes, but he was instrumental in preserving a great deal of the photographic archival material that the rest of us have been 'feeding on' for decades. Plus, as has been stated, he interviewed quite a few old airmen and I (amongst many others) have really benefited from his work.
Marty O'Connor's reference for the green Austro-Hungarian Eindecker color was based on: (A) his interview with
Gottfried Banfield, who had flown several Eindecker purchased for the A-H Navy, and (B) a now-discredited piece of fabric from the Rodney Gerrard collection (I believe Rodney simply made up a piece after Marty may have mentioned the possibility of green Fokker Eindecker to him, and then supplied it to Marty as a genuine piece). If one chooses to put good faith in the esteemed Banfield's recollections (and not everyone does), then go for a green Eindecker.
As Rowan has stated, there is lots of pictorial evidence that some German Eindeckers were not a semi-translucent clear-doped linen. A "L'aerophile" article on Fokker monoplanes stated that, "the fabric of the Fokker wings was generally of beige color in 1916." However, there are British combat reports which indicate darker colors were also evident. McCudden's well-known description of an Eindecker as "a long dark brown form" is one. Another RFC combat report described " a single-seater monoplane, with dark brown wings," and one even described a Fokker Eindecker as "painted a butcher blue with white crosses on it."
Several pilots of the Lafayette Escadrille described having seen an Eindecker painted black, with white crosses! They believed it was flown by Boelcke, but that's unlikely.
On the other hand, there are several contemporary German color paintings of Eindeckers (flown by Boelcke, Immelmann and Bavarian Eduard Boehme) which show the monoplanes as a straw yellow color; it's unknown if the artist ever saw a real Eindecker up close (maybe he just worked from photos?). If I'm not mistaken, there is at least one (and probably more) Henri Farre paintings that show Eindeckers as a yellowish color as well.
So take your pick.
Greg