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 directions. Pour into prepared pan and bake for 25–30 minutes until a toothpick inserted in the center comes out clean.
03 - Remove cake from oven and allow to cool for 10 minutes. Using the handle 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 instant pudding mixes and cold milk for 2 minutes until slightly thickened. Divide evenly among 4 separate bowls. Add a different gel food color to each bowl, mixing thoroughly until vibrant colors are achieved.
05 - Spoon the colored puddings randomly into the prepared holes, creating a multicolored rainbow effect throughout the cake.
06 - Spread the thawed whipped topping evenly over the entire surface of the cooled cake.
07 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping layer.
08 - Refrigerate cake for at least 1 hour to set. Slice into 12 portions and serve chilled.