How far back is too far?

[Steve Ashton]

So ... if you were buying a rugby jersey to wear when attending a match, would you choose one based on your favorite colors ... or would you choose one based on the colors of the team you're cheering for?

I might wear a shirt with my employer's logo on it. I wouldn't wear a shirt with a competitor's logo on it, regardless of the fabric, the cut, the color, or how well designed the logo is.

When I was a student, I wore a letter jacket in my school's colors. I wouldn't have chosen other colors, even though it would have been easy to find colors that I found more aesthetically appealing.

Clothing is used to demonstrate our affiliations. For kilts/tartans, that's frequently a familial affiliation.

While I agree with everything you say, let me turn it around.

There is no rule, not even a convention or expectation that you must wear a team color jersey when attending a footy match.
You can wear any shirt of your choosing.

There is no rule, convention or expectation that someone must wear a letterman's jacket in school colors while at school or later in life.
You can choose to wear any style or color jacket.

I have no Highland Clan or Scottish family association. Just one ancestor who, in the early 1800's, listed on a census form that his father was born in Scotland.
So I wear no Clan affliated Tartans. But I am a kiltmaker, so -

I have a Highland Granite Tartan kilt made by Barb Tewksbury.
I wear the Isle of Skye Tartan just because the colors are so magnificently beautiful.
I wear the Black Watch Tartan, weathered version, for the same reason.
I wear the Victoria, City of Gardens Tartan because I designed it as my gift to my adopted city.
I wear The Company of Adveturers/Hudson' Bay Co. Tartan for, well, I live in Canada.
I wear the USMC Dress Blue Tartan.
And I guess the closest I come to a Clan Tartan is my X Marks Tartan kilt.

[Singlemalt]

I wear the tartan of my surname, usually the hunting version because I like it better.

If I see someone else wearing the same tartan (rare but it has happened). I would certainly make a point of speaking to them.

If we shared the surname I would be pleased and discuss potential blood ties.

If they said it was because of their Great Grandfather on their mother's side I would still be pleased but less interested in seeking a blood relationship which is so far removed.

If they said "I just like the colours" I would be mildly miffed but certainly not enough to say anything unkind. I have no rights of ownership of the tartan any more than I would a street called my surname. (Unless it was named for me but I have yet to do anything notable enough for that.)