Black Watch Tartan 1750's

[Orvis]

Peter - I'm no expert on British Army uniform regulations after the 1780s, and the waistcoat you point out was for an early 1790s fencible regiment, so I presume that the Colonel could have dressed them in red waistcoats had he wanted to (with the approval of the Board of General Officers had the regiment been taken into regular service, as Grant's 97th Regiment was. Note also the lack of regimental lace, which would have been present had the regiment been called into regular service. It was not unusual for waistcoats to have different colored back panels (or in this case, tartan) in the 18th/early 19th centuries, since the waistcoat would have been worn under a regimental coat.

In answer to the earlier question, post-1768 regular regiments which had buff facings did indeed have buff-colored waistcoats and breeches. I'm not sure when this was discontinued, but I presume it was dropped sometime in the early 19th century.