1Heat oven to 350°F (325°F for dark or nonstick pan). Make cake as directed on box for 13x9-inch pan. Cool 15 minutes.
2Mix lemonade concentrate and powdered sugar. Pierce 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. Run knife around sides of pan to loosen cake. Cover and refrigerate about 2 hours or until chilled.
3Spread frosting over top of cake. Sprinkle with colored sugar. Store covered in refrigerator.