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Then I went to an authority in the matter. Greg VanWyngarden of Datafile fame. His comments are presented here.
"Hi,
The model is obviously based on the photos and color profile in the superb book 'Guynemer - Les Avions d'un As' by Bernard Klaeylé and Philippe Osché, with the participation of Christophe Cony (Avions Magazine, Editions Lela Presse 1998, ISBN 2-9509485-6-1). These two authors are acknowledged and highly respected experts in the field of French WWI aviation, with Klaeylé being perhaps THE ultimate authority on French Nieuports (just ask Alan Toelle).
On page 116 they show a color profile of this Nieuport 11 N 836 "Vieux Charles II" from March 1916, in an overall light blue-gray color, and I wouldn't doubt their deductions. The profile is captioned "Avion entièremont bleu horizon" and the English version of the caption says "Overall blue-grey doped aeroplane." The engine cowling is shown as unpainted metal with "Le Vieux Charles" painted on the front surface in blue letters.
On page 104, the description of this aircraft says, in part: "L'avion est neuf et l'entoilage opaque indiquerait qu'il est recouvert d'un enduit coloré, vraisemblablement gris bleu clair". I'll let one of our French forumites translate that, better than give it a try myself - but the meaning is clear.
The aircraft does indeed seem to have an opaque appearance in the photos. Interestingly, another Nieuport 16 of Escadrille N62 on page 42 is camouflaged, and the caption says "Colors are thought to be green and brown sprayed over the former blue shade."
These authors really put a tremendous amount of painstaking research into their conclusions, and I-for one- am not prepared to dismiss their statements readily...Greg"