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 1750s.
