As an 18th century British Regular reenactor, if like to add my tuppence worth on the matter (though I portray a little later in the century).
If you wanted these to look like they made their capote blanket coats out of their issued blankets, you would want to go with an cream coloured wool. Blankets carried by British regulars at the time were undyed wool, sometimes with a blue stripe through the middle, and a small 3in GR stamp in a corner. Period depictions of blanket-coats often show clever use of that blue stripe in the final garment, at the hem of the skirt and) or on the cuffs.
Of course locally sourced blankets would likely be Hudson Bay Company and have multiple-colored stripes.
I hope this is helpful!