that's why i'm still hesitating....its still a sin to cut old rare parts but maybe GB will reissue white ones
...and youre right.....all drawings seems to show their back-covers as white 
Hi Cheng your "Ashigaru hats" (jingasa) look fantastic!
As for the cloth hanging from the hats, I'm not sure what the origin or nature of the material was. I suspect these were linen cloths worn for protection from the weather or thicker cloth in mock-up of the neck protection of the samurai helmets.
In any event, I don't think white was "a uniform standard" (if such a thing even existed) so whatever color you have on hand looks great! (and probably reflects the reality--the cloth might even have been rags the soldiers mothers sewed for them!)
Tim