Many clans didn't had their own tartan when tartans and kilts became a (upper class) hype early 19th century, so they went to Wilson of Bannockburn...the most important weaver.
Some picked a tartan from an old painting, others had their tartan designed and some just picked an excisting tartan from a sample book....and claimed it. Some tartans changed name this way (The Regency tartan became the MacLaren tartan for instance).

And to make it all a bit more complicated...the Sobieski brothers came up with the "Vestiarium Scoticum" with "ancient" (clan) tartans...they invented themselves :rolleyes:
The "yellow" MacLeod is a Sobieski invented tartan for instance.

Conclusion...clan tartans are not as old as many think they are, some tartans were in use as fashion (because clans just didn't had a tartan) tartans before they became....clan tartans.

What a mess

So...wear the tartan you like, you can't make it any worse