I have used a combination of gray en silver for my Pfalz planes to get a good silbergrau, if the Nieuports and other french planes were a silver dope, I would not add grey to the mix.
I think I will be using Tamiya paints, either their silver or aluminium and finish off with a semi-gloss coat.
Other 'real' metal paints are too bright for a dope finish, the grains in the Tamiya paints should be be just fine.
anyway, just my 2 cents..
Hidde