And yet Matt Newsome also says:

Originally, tartan designs had no names, and no symbolic meaning. All tartan cloth was hand woven, and usually supplied locally. While it may have been true that certain colors or pattern motifs were more common in some areas than others, no regulated or defined "clan tartan" system ever existed.
I'm not saying that there was a strictly regulated "clan tartan" system as there is in modern times. But there does seem to be plenty of evidence for a generally practiced system of tartans held to be customary for a particular region.