Blue is the original color. St. Patty when he returned to Ireland after accepting his priesthood, wore blue garments.
Also, St. Patty was not Irish. He was born British but as a young lad, he was kidnapped into slavery to work in Ireland.
Just thought I would share.
Green represents the Catholics of Ireland. The ones that celebrate Saintsā¦and they probably arenāt prejudice that heās British. He did banish the snakes from Ireland :-\ Just sayin.
I thought Green represented the Emerald Isle as Ireland is named?
St. Patty never banished any snakes there. Plain old legend.
St. Pat Morita banished them. Banish all Cobra Kai.
HereĀ ya go. The great wiki speaks.
And BLUE it was.Ā 
Itās kind of odd that in America we celebrate St. Patricks day instead of Evacuation Day on March 17.
Gasp!
Iām just throwing this in there, but, to use the name St. Patty, you are actually referring to the feminine, Patricia. The āproperā spelling is St. Paddy which is short for Padraig (or Padraic), the Irish version of the masculine, Patrick. I mean, as long as weāre pointing out stuff. 
Potato Famineā¦
It doesnāt matter that it was blue. Weāve Americanized it. That along with all the other āculturalā holidays we celebrate.
American Pizza/Chinese food FTW
Iām one of the few people who donāt celebrate this holiday. I refuse to honor a man who was partly responsible for destroying a large portion of celtic culture and tradition. When people talk about āchasing the snakes out of irelandā it refers to the fact that he, along with british soldiers, exiled or killed celtic pagan druids, and basically outlawed druidism and the use of gaelic language.