Why do you were green on st Patrick?

St. Patrick used a three-leaf clover to explain the mystery of the trinity. The clover, or shamrock then came to symbolize St. Patrick. Clovers are usually very green. Surprisingly, the traditional color for St. Patrick's day is not green but blue. However, over the years, the blue faded and green became the most popular color.
info: http://en.wikipedia.org/wiki/Saint_Patrick's_Day#Wearing_of_green