Found this blurb today while perusing the interwebs. I've heard of the green and orange being catholic and protestant respectively, but never purple for paganism. I wonder how true it is.
"During the 1st through 5th centuries during the Roman Empire expansion into Gaul and other Celtic lands, the Druids were not completely included into the new oligarchy. Julius Caesar in the first century even campaigned against them. Tiberius even passed laws of prohibition against practicing druidism. However early writing at the time gave very high social standing to these "priests." Their ceremonial dress often included green and purple.
The rise of the Catholic Church and eventual turn of the Roman Empire into the Holy Roman Empire where the Pope because more powerful than the Caesars re! sulted in what became known as the Spanish Inquisition (as the power center moved from Rome to Spain). As the Catholic Church worked to eradicate the other religions (Paganism, Druidism, Judaism, Islam, and eventually Protestantism) the Pagans and especially the Druids began to use purple dyed clothes and purple face paints in their usually defensive skirmishes. Thus from the 12th century until the late 18th century, purple stained faces and fingers and clothes became to be associated with paganism.
The purple dye came from woad, and the indigo process was usually associated with lower classes due to the smell of the process and the health hazards. By the 13th century the travails of "miller's blue" were subject of laws and eventually reforms in thew 19th century industrial revolution.
Roughly one can make the argument that the Irish flag shows the religious separation of the country: green = Catholic, orange = Protestant. Purple is not present but would represent the pagans. Wearing purple on St. Patrick's Day is a rather obscure tradition in Wiccan and other pagan observances."