Hi Albert, I guess the yellowing would depend on whether the airframe had been in service long, which as I'm sure you know, was a chancy thing in WWI. Heavily weathered Nieuports are unusual from the photos I've seen, aside from burnt castor oil from the rotary and dirt & mud on the undersides (except for some of the two-seaters and a lot of the Russian a/c) so I'd probably go with no aging on the dope.
I'm looking forward to seeing what your experience is like with the clear Aviattic linen-effect decals, as I got some to use on my Nieuport 16, which is going to have a three-color (or possibly four) camo scheme, with pale blue (or possibly CDL, or possibly grey... :-P) undersides.
Looking forward to seeing how you make out!
Dutch