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 until a toothpick inserted into the center comes out clean.
03 - Remove cake from oven and allow to cool for 10 minutes. Using the handle of a wooden spoon, create holes every 1-2 inches across the surface of the warm cake.
04 - In a large mixing bowl, whisk together both packages of instant vanilla pudding mix with cold milk for approximately 2 minutes until mixture begins to thicken.
05 - Divide pudding mixture evenly among 4 separate bowls. Add a different gel food coloring to each bowl, mixing thoroughly until vibrant colors are achieved.
06 - Spoon the colored puddings randomly into the poked holes, alternating colors to create a rainbow effect throughout the cake.
07 - Spread thawed whipped topping evenly over the entire surface of the cooled cake.
08 - Sprinkle unicorn or confetti sprinkles generously over the top. Refrigerate for at least 1 hour before slicing and serving to allow flavors to meld.