Captain, this is just me spouting off, and I think the man with the answer you're looking for is named "Matt Newsome." With that ritualistic belittling of what I'm about to say out of the way, here's my take. A tartan isn't defined so much by it's shade, as by it's thread count. The "Campbell ancient" philabeg I have is, by thread count, indistinguishable from Black Watch, aka Gov't Sett, aka etc. etc. ad infinitum. The color is quite different.

Sett size, and the "shade" of color, is really . . . unimportant? Negligible? Going to a thicker yarn will give a larger sett, and you could theoretically put a "2x" in front of every thread count . . . which would give you a much larger sett. Witness, on color, the variations between how "modern" and "ancient" and "weathered" look from the various mills.

I hope I've quelled, to a degree, the paranoia. (And remember: paranoia can be your friend! It has been to me on many an occasion!) With luck Matt, or perhaps Cajunscot, will weigh in on this matter.

I am but an enthusiastic amateur, but I wouldn't worry about the difference in either sett size, or shade of thread. And I think Strathmoore is a pretty reputable vendor, to boot. (Not positive, but that's my impression.)