Wearing green and leprechaun costumes on St. Patrick's Day are traditions that started in the 17th century, when people wore green ribbons and shamrocks on March 17 to honor Ireland's patron saint. The tradition was popularized by Irish immigrants in the United States, who believed that wearing green made them invisible to leprechauns, fairy creatures who would pinch anyone they could see. Pinching people who didn't wear green reminded them that leprechauns could sneak up on them at any time.
Of course, green wasn't always the iconic color of the holiday. Originally, it was blue! Green was thought to have flourished as a tribute to the Emerald Isles and inspired the color change from blue (back in the 17th century) in order to better match the Irish flag.
But for a growing number of people, taking part in the holiday means wearing orange. According to this increasingly popular tradition, Protestants wear orange and leave green attire to Catholics. Thus, the color you wear actually depends on your religious affiliation. While this color tradition is not well known, it has deep roots in Irish history.