To be fair, many of those Stewart tartans are really fashion tartans. Camel, navy, golfing, grey, dress blue and even (arguably) black Stewart are fashion tartans. Problem is, as far as I can tell, our clan chiefdom resides in Her Majesty the Queen, and she does not make pronouncements on our clan tartans. I reckon that old sett, royal, hunting, dress and the local (Stuart of Bute for example) tartans are the ones that you could call "official." Of course, not every Stewart would be entitled to wear the Bute tartan.