I think anything from White through to Mid grey will be ok. I don't believe there was a regulation colour at this stage and the various sources I have read are not particularly prescriptive. Interestingly when browsing through the details in Sapherson's Dutch Army of William III any random entry can be describe thus:
Grey coat faced red(Source A)
White coat faced red(Source B).
This would be for the SAME unit and is obviously an issue of interpretation.
I think the question is how do you define `white'(offwhite, bleached wool, brilliant dulux white???)
My suggestion is to avoid `flat' black and white mixes and go for :
Mix of medium flesh and black - as initial base coat - lighten first time with more flesh and then after with white. This gives a very interesting greyish shade.
Try also working up from a blueish grey to grey white.
Another tip would be to make your final highlight `off white' such as Vallejo Ivory
or Workshop Bleached bone
I think this is worth a QND Guide.
On the flags: Ref against QND Guide on Flag making but the dims are 50 mm in the hoist, 53mm in the fly.
Hope this helps