The reason why guys wear green on St. Patrick's Day is because green is one of the colors in Ireland's tri-color flag. It used because Ireland has lush green landscape. It is a cultural and religious holiday celebrated on 17 March. Visit http://en.wikipedia.org/wiki/Saint_Patrick's_Day for additional details.
Saint Patrick converted many of the Irish to Catholicism by comparing the Holy Trinity to the Shamrock (3 parts to the same leaf). Although the original color was blue, the color was changed to green as he was best known by the shamrock, which was green.
The color traditionally associated with St. Patrick was blue. The emphasis on green is thought to be linked to "the wearing of the green," which meant to wear a shamrock, symbolic of the Trinity. ......... yes, I copied and pasted this from the question of the day..... :)