Place popcorn in a large baking pan; keep warm in a 200° oven. , In a heavy saucepan, combine the sugar, corn syrup, water and salt. Cook over medium heat until a candy thermometer reads 235° (soft-ball stage). , Remove from the heat. Add the butter, vanilla and food coloring if desired; stir until butter is melted. Immediately pour over popcorn and stir until evenly coated. , When mixture is cool enough to handle, quickly shape into 3-in. balls, dipping hands in cold water to prevent sticking.