01 - Preheat oven to 350°F. Generously coat a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter according to package instructions. Pour into prepared pan and bake for 25-30 minutes until a toothpick inserted in 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 surface of the warm cake.
04 - In a large bowl, whisk together pudding mixes and cold milk for about 2 minutes until mixture begins to slightly thicken.
05 - Divide pudding evenly among 4 separate bowls. Add a different gel food color to each bowl, mixing thoroughly until vibrant colors are achieved.
06 - Spoon colored puddings randomly into the poked holes, alternating colors to create a rainbow effect throughout the cake.
07 - Spread whipped topping evenly over the entire surface of the cooled cake using a spatula.
08 - Sprinkle unicorn or colorful confetti sprinkles generously over top. Refrigerate for at least 1 hour before slicing and serving to allow flavors to meld.