It is important to understand that historically, there is no such thing as an accurate reference to ancient tartans. All the references to tartan patterns that we have today were made up after the 1800's, primarily during the Victorian era. Prior to that, scotch music, dress and culture was severly censured because of English fears of further Jacobite rebellion after 1745. As a result, there is no scholarly evidence that the patterns of scottish clan tartans predate this period. Essentially, you can make it up completely, and no one can definatively say whether you're right or wrong. Just avoid non-period colours like neon green or orange, and you'd be fine.