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 in the center comes out clean.
03 - Remove cake from oven and let cool in the pan for 10 minutes.
04 - Using the handle of a wooden spoon, poke holes every 1–2 inches across the surface of the warm cake.
05 - In a large bowl, whisk together instant pudding mixes and cold milk for about 2 minutes, until mixture begins to thicken slightly.
06 - Divide pudding evenly among 4 separate bowls. Add a different gel food coloring to each bowl and mix until vibrant colors are achieved.
07 - Spoon the colored puddings into the holes randomly, alternating colors to create a rainbow effect throughout the cake.
08 - Spread thawed whipped topping evenly over the entire surface of the cooled cake.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping.
10 - Refrigerate cake for at least 1 hour to allow pudding to set. Slice into squares and serve chilled.