I researched about the day and apparently (according to Wikipedia) the original colour associated with St.Patrick's day was actually blue, not green!
It eventually became green after shamrocks (three leaf clovers) were worn to explain the Holy Trinity on the back-then religious holiday. Eventually, shamrock-inspired designs and green clothing were worn. Now, we even have green beer.
On my way to the subway, I saw my very first St.Patrick's day parade :).