I wish to clear some misinformation that has spread on this board. Clan Muir is not an official sept of the House of Gordon. Please read the Gordon sept list:
Clan Muir is an official sept of two clans:
Plus there is a Clan Muir society
Here is the wikipeida article about Clan Muir. The chief of the House of Gordon removed the Muir septs except for the Moir spelling variation.
Once a chief removes your surname, you are not supposed to wear his tartan. I have stopped wearing the Gordon tartan. I have one kilt in the Pride of Scotland (purple) tartan. This is explained in the following article:
The tartan dealers or kiltmakers will not tell you when names are removed because they want to make money. Personally, I do not wear a tartan, unless my name is an official sept, that is approved by the chief.