Green beer is an American St. Patrick's Day tradition. You can buy kegs of green beer from your local liquor store, or you may choose to add your own green coloring. But beer cannot be dyed while in the keg unless the color is added during the brewing process. Add a non-toxic, natural food coloring. Blue dye is ideal for making green beer. When the dye is dispensed into yellow beer, it turns a nice shade of green. Lighter beers have the best results. Darker beers tend to keep their normal color and only the foam "head" of the beer is colored green. A health food algae called spirulina can be substituted for the blue dye. It is a natural green coloring that works best on light colored beers.
Brew a keg of light-colored beer.
Add food coloring using the syringe provided with the dye injector kit. Brewers recommend adding about three ounces of dye for a keg of beer.
Pressurize the keg to prevent beer from getting flat. Home brewers can do this themselves, but the equipment for pressurizing can be expensive which is why most home brewers choose to bottle their beers. Another option is to take the keg to a local brew master or retailer to have it pressurized. This typically costs around $10.