Green Beer is beer that has undergone its primary fermentation but has yet to undergo a period of conditioning before packaging. It is perhaps “drinkable” but not ready to drink.
Before it can leave the brewery the beer will require a period of maturation, which, depending on the beer, can be the longest part of the brewing process. This may be as little as a few days for some British cask beers or as long as a few months for very traditional Czech pilsner.
Green beer is often cloudy with unsettled yeast, and in most cases it will eventually need to be filtered or clarified in some fashion unless it is of a type that is meant to be served hazy. Generally speaking, beer is no longer considered “green” when it has reached full maturity of flavor and aroma.
Of course in some parts of the world, cheap industrial beer may be artificially dyed green for St. Patrick’s Day; such things, although harmless, are not to be contemplated by beer enthusiasts.