Bear in mind that the drummers of regiments with white facings would also have worn red "small clothes" (ie waistcoat and breeches) as well; in the case of a "kilted" regiment, like the 71st, this would have been waistcoat only and the overalls (leggings in winter) would have been the same colour as those of the rank-and-file, as seen in the illustration.
Note that the pipers wore red coats as they were actual soldiers from the ranks who were paid a bit extra by the officers, or perhaps from regimental funds, to do this as an extra duty, whereas the drummers/fifers would have been specialists and listed as such in the twice-yearly Muster Rolls.
The 71st appear to have worn their kilts only in the first year or so of their time in America (ie 1776), and adopted overalls from that point on, as did the 42nd. Other kilted regiments, which arrived later in the war, also seem to have adopted overalls quite quickly.