Originally the color associated with Saint Patrick was blue. However, 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. He is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pre-Christian Irish, and the wearing and display of shamrocks and shamrock-inspired designs have become a ubiquitous feature of the day.
and remember, "An Irishman is never drunk as long as he can hold onto one blade of grass to keep from falling off the earth."