While my favorite kilt is my Ancient MacKay both because it's my Clan tartan and because it's got my favorite colors in it. (By an odd coincidence I have always liked a particular shade of blue and of green, and it turns out that those exact colors are also the predominant colors in the Ancient MacKay! Who'd have thought that a tartan could be genetic!)

However, I also frequently wear a Cunningham in honor of my Grandmother's clan and also because it has the same colors as my old Fire Department colors.

Personally, I stick with tartans with which I have some connection or which are general tartans, but that's might change as I increase my kilt collection.

And as for not being a part of a particular Clan by name or by Sept, consider this. Septs of a clan came about by individuals of no blood relation placing themselves under the protection of a Clan, or by swearing fealty to the Chief.

Theoretically, you could still use that Medieval method if you feel that you absolutely, positively must have some kind of connection to the MacKays before you can wear our tartans.

I would say to try contacting Lord Raey (Chief of the MacKay Clan) but recently the MacKay Clan Association of the US (http://www.clanmackayusa.org/) tried to get his permission to register a new Dress MacKay tartan for the use of the entire Clan and he never responded. So instead it is now being registered as a proprietary tartan of the US MacKay Clan Association.