Ethics and courtesies of branching into other tartans?

[Tobus]

The concept of clan tartans came about in the early 1800s during the romantic revival period where all things Scottish were popular. Prior to Culloden in 1746, Highlanders wore a wide array of tartans, very few of which held any real significance. In essence, they were fashion tartans, chosen for their looks and not much else. So yes, feel free to pick any fashion tartan and wear it if you like it. It's an ancient Highland custom.