This is posted at reference.com (scroll down, it is the paragraph just above the Related Articles section):

"Children in the U.S. celebrate St. Patrick's day by wearing green colored clothing and items. Traditionally, those who are caught not wearing green are pinched, leading to several St. Patrick's Day items hosting phrases such as "Can't pinch me!" Of course, people caught wearing orange (the colors of the opposing group Orange Order) on this day are always suggested to be wary, regardless of what country they're living in."