Heat oven to 350°F. Prepare cake mix as directed on package. Divide batter evenly between greased and floured 8" round and 8" square baking pans. Bake as directed on package. Cool 10 minutes; remove from pans. Cool completely on wire racks. Pour milk into medium bowl. Add pudding mix and sugar. Beat with wire whisk 2 minutes. Gently stir in whipped topping and food coloring. Leave square cake whole; cut round cake in half. Assemble pieces by turning square cake to diamond shape and using small amount of pudding mixture holding each half of round cake to the top sides of diamond sides, making a heart shape. Frost cake with remaining pudding mixture. Decorate with candies or sprinkles.