01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter according to package directions. Pour batter into prepared pan and bake for 25-30 minutes, or until a wooden toothpick inserted into the center comes out clean.
03 - Remove cake from oven and allow to cool in the pan for 10 minutes.
04 - Using the rounded end of a wooden spoon, poke holes every 1-2 inches across the surface of the warm cake, creating a grid pattern.
05 - In a large mixing bowl, whisk together both packages of instant vanilla pudding mix with cold milk for approximately 2 minutes, until mixture begins to thicken slightly.
06 - Divide pudding mixture evenly among 4 separate bowls. Add a different gel food coloring to each bowl and stir until vibrant colors are achieved.
07 - Spoon the colored puddings into the prepared holes randomly, alternating colors to create a rainbow effect throughout the cake.
08 - Spread the thawed whipped topping evenly over the entire surface of the cooled cake using a spatula.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping.
10 - Refrigerate cake for at least 1 hour to allow flavors to meld and pudding to set. Slice into squares and serve cold.