The original flag of Parthia was simply Parthian blue (light blue, as can be seen on the flag, the color of Parthia) with a purple crown in the middle. This crown perhaps was purple in honor of their former Seleucid masters, seeing as Parthia (and Bactria too) had been vassals of the Seleucid Empire until somewhere around 300 BC, but perhaps it simply was purple because that was and is the color of kings and queens and the likes. File:Seleucid parthia.png