St. Patrick's Blue, not green, was the colour long-associated with St. Patrick. Green, the colour most widely associated with Ireland, with Irish people, and with St. Patrick's Day in modern times, may have gained its prominence through the phrase "the wearing of the green" meaning to wear a shamrock on one's clothing. At many times in Irish history, to do so was seen as a sign of Irish nationalism or loyalty to the Roman Catholic faith. St. Patrick used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pre-Christian Irish. The wearing of and display of shamrocks and shamrock-inspired designs have become a ubiquitous feature of the saint's holiday. The change to Ireland's association with green rather than blue probably began around the 1750's.
Some Protestants have begun wearing orange on St. Patrick's Day as a mark of defiance. This relatively new tradition has its roots in William of Orange (William III), the King of England, Scotland, and Ireland, who defeated King James II, a Roman Catholic, in the Battle of the Boyne near Dublin. William's victory would ensure Protestant military dominance on the island and has been a source of tension ever since. Although the "Orange" in William's name actually referred to a province in southern France, the colour reference of orange for Protestants stuck. This is why orange now appears in the Irish flag - to symbolize the Protestant minority in Ireland. The first group to take part in the tradition of wearing orange on St. Patrick's Day appears to have been the Orange Institution, a Protestant fraternal organization more commonly known as the Orange Order. Some members of the order wore orange in various parades on St. Patrick's Day as a mark of defiance.