Irish citizens do not, to the best of my knowledge, wear any particular color on the feast of St. Patrick. It’s a religious holiday which only recently began to take on the silly drunken overtones of the American celebrations (granted, imbibing in Ireland is not frowned upon for any occasion).
I find it peculiar that "the wearing of the green" has become such a fixture for Americans of Irish ancestry and many who have no such ties to the country. Just a case, I suppose, of Americans wearing their hearts on their sleeves, or chests or backsides, in this instance.
In grade school, since St. Joseph’s day follows St. Paddy’s by two days, and many of us were of Polish or Italian ancestry, we kids felt compelled to wear red and white attire to honor St. Joseph on his day. Thinking back on this behavior made me why we did this. Interestingly (to me at least), I found this:
St. Joseph’s Day in Chicago Polonia: A Polish-American Hybrid
While Poles most certainly honor and revere St. Joseph, in American Polonia these values have flourished in interesting and hybrid ways. Especially in the earlier waves of immigration (1890s - 1930s), Polish and Italian immigrants were faced with an American Catholic church hierarchy controlled largely by Irish clergy, most often unsympathetic to the newcomers whom they often regarded as inferior, primitive, overly demonstrative, and superstitious. In the face of this disdain for Southern and Eastern European Catholicism, Poles responded by forming their own Polish language parishes (i.e. St. Stanislaus Kostka in Chicago) while Italians responded by preserving their religious traditions in the form of "Feasts" (Festa) run by patronage societies from their home villages and cities. This tension found curious expression in Chicago, America’s largest Catholic Archdiocese.
Run by a largely Irish political and church elite, the city visibly celebrated the Feast of St. Patrick on March 17. In Chicago, this included a prominent parade and turning the Chicago River green. In multi-ethnic parochial schools this found expression in "the wearing of the green" visually marking those of Irish heritage. As is often the case in diasporic immigrant culture, the importance of St. Joseph’s Day escalated and found new significance in a new context. In immigrant Polish and Italian communities this provided an alternative form of cultural identification and expression of loyalty. Just two short days later, Polish and Italian Americans dressed in red, celebrating their patron and publicly showing their ethnic identity. (Both national flags include this color as opposed to the Irish green). Especially in those of school age, this created a curious linking of the two ethnic groups, who identified against the traditions of the Irish.
from here http://acweb.colum.edu/users/agunkel/homepage/easter/swjozef.html