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 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 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 packages of instant vanilla pudding mix and cold milk for about 2 minutes, until mixture begins to thicken slightly.
05 - Divide pudding evenly among 4 separate bowls. Add a different gel food coloring to each bowl (pink, purple, blue, yellow) and stir until vibrant colors are achieved.
06 - Spoon the colored puddings into the holes randomly, alternating colors to create a rainbow effect throughout the cake.
07 - Spread the 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 top. Refrigerate for at least 1 hour to allow pudding to set before slicing and serving.