01 - Preheat oven to 350°F. Generously coat a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter following 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 let cool in the pan for 10 minutes. Using the handle end of a wooden spoon, poke holes every 1–2 inches across the surface of the warm cake.
04 - In a large mixing bowl, whisk together instant vanilla pudding mixes and cold milk for approximately 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, mixing thoroughly until vibrant colors are achieved.
06 - Spoon the colored puddings randomly into the holes, alternating colors to create a rainbow effect throughout the cake.
07 - Spread thawed whipped topping evenly over the entire surface of the cooled cake using a spatula.
08 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping. Refrigerate cake for at least 1 hour before slicing and serving to allow flavors to meld.