Dressing in the color green is one of the most popular ways people celebrate Saint Patrick’s Day. It pays tribute to the country of Ireland, whose landscape remains green year round, earning it the poetic name of “The Emerald Isle”. Wearing green, especially on Saint Patrick’s Day, is believed to bring a person good luck.
Originally, the color associated with Saint Patrick was blue. Over the years the color green and its association with Saint Patrick's Day grew. Green ribbons and shamrocks were worn in celebration of St Patrick's Day as early as the 17th century. Saint Patrick is said to have used the shamrock to explain the Holy Trinity to the pagan Irish, and the wearing and display of shamrocks and shamrock-inspired designs have become a universal feature of the day.
In the 1798 rebellion, to make a political statement, Irish soldiers wore full green uniforms on March 17 in hopes of catching public attention. The phrase "the wearing of the green" means to wear a shamrock on one's clothing. So the wearing of green is a symbol of Saint Patrick's Day and then Ireland, the Emerald Isle, in general. The day as a holiday with parades, green beer etc. is more of an American tradition than Irish, and of course everyone is Irish on St. Paddy's day.