Your instincts are correct.. there are quite a few websites on the history of tartan and the dyes of place. Its not called plaid over here, that is actually the garment that is wrapped around, it is tartan; but the meaning of that word is not yet sure. Irish French Spanish sources have been put forward.
"Tartan only started to be used as a means of Scottish clan identification during the 19th century, but historians believe that in ancient times people could tell what area of Scotland someone was from by the shade of the dyes that were used in their clothing.
Natural dyes were produced from lichen, bark, or berries of plants and trees. Because the weavers were restricted to the colours they could produce from the local vegetation, tartans produced in the area were frequently made in the same colours and frequently even in the same pattern.
In the 18th century, a Scottish writer called Martin Martin wrote in his Description of the Western Isles of Scotland that "Every isle differs from each other in their fancy of making plaids as to the stripes in breadth and colours. This humour is as different through the mainland of the Highlands, in so far that they who have seen those places are able at first view of a man’s plaid to guess the place of his residence."
Similarly, when in the late 18th century Sir Alan Cameron of Erracht founded the Cameron Highlanders regiment, he designed a red tartan for it on the basis that the traditional tartan worn in the Lochaber region was red in colour.
During the 19th century, tartan became widespread in Scotland as a means of Scottish clan identification. Some authors maintain that -like Sir Alan Cameron- many clan chiefs might have originally adopted their tartan based on the shade of dyes that were most commonly used in their district."
http://www.tartan.galician.org/history.htm
http://www.tartansauthority.com/tartan/the-growth-of-tartan/tartan-production/colours-and-dyeing/
http://www.shilasdair-yarns.com/
