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 instructions. Pour batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
03 - Remove cake from the oven and let it cool for 10 minutes in the pan.
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 mixing bowl, whisk together the instant vanilla pudding mixes and cold milk until the mixture begins to thicken, approximately 2 minutes.
06 - Divide the pudding evenly among 4 separate bowls. Add a different gel food coloring to each bowl and mix thoroughly until vibrant colors are achieved.
07 - Spoon the colored puddings randomly into the poke holes, creating a rainbow effect throughout the cake.
08 - Spread the thawed whipped topping evenly over the entire surface of the cooled cake.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over the top. Refrigerate for at least 1 hour to allow flavors to meld before slicing and serving.