Heat oven to 350°F (325°F for dark or nonstick pan). Make cake batter as directed on box. Colour batter with red food colour to reach desired shade of pink. Bake cake as directed on box for 13x9-inch pan. Cool 10 minutes. Run knife around sides of pan to loosen cake; remove from pan to cooling rack. Cool completely, about 1 hour. Place cake in freezer 30 minutes before cutting to reduce crumbs.