How to Make It
Prepare the Cake: Preheat oven to 350°F. Prepare cake mix according to package directions, replacing water with whole milk and adding vanilla extract. Pour batter into a greased (with shortening) and floured 13- x 9-inch baking pan.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Cool in pan 15 minutes. Use blunt end of a wooden skewer to poke holes in cake at 1-inch intervals.
Meanwhile, bring 1 cup water to a boil in a small saucepan; stir in gelatin until completely dissolved, about 2 minutes. Stir in remaining 1/2 cup water. Pour gelatin mixture evenly over warm cake, letting it run down into holes. Cover and refrigerate until chilled, about 3 hours.
Spoon dollops of each color frosting over top of cake; use a small offset spatula to swirl frostings together in a tie-dye pattern, covering surface of cake. Garnish with candies as desired.
If desired, use 2 (16-oz.) containers white frosting instead of colored canned frostings. Divide frosting into 3 bowls; tint each bowl separately with 1/2 teaspoon gel food coloring (sky blue, hot pink, and bright purple).