French uniforms were made of undyed wool so they could be any thing from medium Gray to white. I generally use Valijo(Spelling?) Silver Gray but I am painting for WAS and 7YW a some what grayer gray might be better for the earlier war.
Coats could vary some times form regiment to regiment because of variations in the supply of wool. The French government made an effort to bread whiter sheep as the 18th century advanced.
Thanks Adam I actually saw some other discussions about this. I am planning to paint my offwhite possible with some very light cream tones. I am painting War of Spanish succession and many of the figures that I see from this period seems somewhat darker grey. But I have seen uniforms that look almost white too.