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 instructions. Pour batter into prepared pan and bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
03 - Remove cake from oven and let cool in pan for 10 minutes.
04 - Using the handle end of a wooden spoon, poke holes every 1–2 inches across the surface of the warm cake, pressing about halfway through.
05 - In a large bowl, whisk together instant vanilla pudding mixes and cold milk until mixture begins to thicken, approximately 2 minutes.
06 - Divide pudding evenly among 4 separate bowls. Add a different gel food coloring to each bowl and mix until vibrant colors are achieved.
07 - Spoon the colored puddings into the holes randomly, creating a rainbow pattern across the cake. Work quickly before pudding sets completely.
08 - Spread whipped topping evenly over the entire surface of the 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 pudding to set completely. Slice into squares and serve chilled.