01 - Preheat oven to 350°F. Coat a 9x13-inch baking pan generously with non-stick cooking spray.
02 - Prepare cake batter according to package directions. Spread batter evenly in prepared pan. Bake for 25–30 minutes until a toothpick inserted into the center comes out clean.
03 - Remove cake from oven and let cool for 10 minutes. Using the handle end of a wooden spoon, poke holes every 1–2 inches across the entire surface of the warm cake.
04 - In a large bowl, whisk together both vanilla pudding mixes and cold milk for about 2 minutes until mixture begins to thicken slightly.
05 - Divide pudding mixture evenly among 4 separate bowls. Add a different gel food coloring to each bowl, stirring until you achieve vibrant, saturated colors.
06 - Spoon the colored puddings randomly into the poked holes, alternating colors to create a rainbow pattern throughout the cake.
07 - Spread the thawed whipped topping evenly over the entire surface of the cooled cake using a spatula.
08 - Generously sprinkle unicorn or colorful confetti sprinkles over the top. Refrigerate for at least 1 hour before slicing to allow flavors to meld and pudding to set.