Peter,

Interesting reminder about the original Balmoral tartan being a tweed.

To further your comments, though, I have never viewed the terms "tweed" and "tartan" as being exclusive.

I see tweed as being a type of cloth, specifically a rough woolen fabric made using mixed yarns, as you describe. Tartan, on the other hand, can be used to refer to both the pattern as well as cloth woven in the pattern. So it is entirely possible to have tweed cloth woven in a tartan pattern, and it ceases to be neither tweed nor tartan. The two terms are not exclusive.

If one takes a close look at this image, showing Harris Tweed cloth in the Black Watch tartan, with the Grant/Drummond tartan in the overlay, you can see that the "solid" color yarns are in fact a color mix - a true tweed. That's a large part of what gives the tweed cloth its beautiful character.