This St, Patricks Day, as millions around the world don their green clothing, those in Ireland will be putting on some blue. Traditionally, St. Patrick wore a blue cloak and hood and the Irish that live here on the island continue this custom.
The "wearing of the green" comes from the practice of pinning a shamrock on your lapel. Centuries ago, wearing a shamrock was considered rebellious and even illegal. In some parts of the world, to say you're "wearing the green" meant you sided with the Irish rebellion against the English rule.
Why the lapel custom? Legend says that in the 5th century, St. Patrick used the three leaf clover or "seamrog" to explain the triad of Chistianity to the Druids. The Celtic religion considered the number three to be mystical and sacred, sometime referring to totality, or the sky, earth, and underworld.
Today, the shamrock is often confused with the four-leaf clovers, but the two have differen meanings. The four-leaf clover symbolizes good luck. Shamrocks, on the other hand, are simply a native plant of Ireland and represent Irish pride and tradition. -http://americanirish.blogspot.com/