Green beer is not a true style of beer, but an American serving tradition associated with the St. Patrick's Day holiday. Beer, usually standard American pale lager, is dosed with green food coloring, resulting in a greenish-yellow hue.
Due to a massive marketing campaign in the United States, dry stout, specifically Guinness, has supplanted green beer as the standard St. Patrick's Day drink in the United States.
Although it is not what is normally thought of as "green beer", Berliner Weisse can have a similar greenish color when it is served with traditional woodruff syrup.
