"St. Patrick's Blue" is still used for both Ireland's Presidential flag and on the plumes of the Irish Guardsmen's hats. Unfortunately, blue was up against some difficult competition in maintaining its associations, not only to the saint, but also to the rest of the Irish nation! Green is the color of the Irish countryside and the reason Ireland has the nickname the "Emerald Isle."
St. Patrick himself used a green shamrock to represent the Catholic Holy Trinity. The wearing of these plants on the holiday eventually grew into a single hue being representative of Irish Catholics politically. The 'wearing o' the green' was a powerful symbol during Ireland's quest for independence and continues to be practiced on March 17th each year throughout the Western world.
If you're still in the holiday season for whatever reason, check out this this list of St. Patrick's day facts.
There's also this History Channel video about the color green's connection to St. Patrick's Day.