I think some of the newer 'commissioned' (state, corporate, school, etc.) tartans have some meaning ascribed to the colors. (Probably to answer the question: "Why did you choose THAT color for our tartan?") This would help if there needs to be an official OK to have a mill produce some. I know there's meaning ascribed to the colors in the Kentucky tartan.

As far as the Scottish clans go, so far as I know, and as Jock said, there's no meaning whatsoever (unless, as you suggested, it's a back-invention for the romantic interest). From what I've read (here & elsewhere), before the Georgian/Victorian revival of tartan in the 1800's, most tartans were fairly simple, made from the dyes the weavers had avaiable.