01 - Preheat oven to 350°F. Spray 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, or until a toothpick inserted into the center comes out clean.
03 - Remove cake from oven and let cool in the pan for 10 minutes.
04 - Using the handle end 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.
06 - Divide pudding evenly among 4 separate bowls. Add gel food coloring to each bowl, creating pink, purple, blue, and yellow shades. Mix until colors are vibrant.
07 - Spoon the colored puddings into the poke holes randomly, distributing all four colors across the cake to create a rainbow effect.
08 - Spread the thawed whipped topping evenly over the entire surface of the cake.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over the whipped topping.
10 - Refrigerate the cake for at least 1 hour to allow flavors to meld and pudding to set. Slice into squares and serve cold.