Allow cake to cool for 15 minutes. Stir together lemonade concentrate and powdered sugar. Poke top of warm cake every 1/2 inch with long tined fork, wiping fork occasionally to reduce sticking. Drizzle lemonade mixture evenly over top of cake. Cover and refrigerate about 2 hours until chilled. Spread frosting over top of cake. Sprinkle with colored sugar, if desired. Store loosely covered in refrigerator.