01 - Preheat oven to 350°F. Prepare a 9x13-inch baking pan with non-stick spray.
02 - Prepare the cake batter according to package instructions. Pour into the prepared pan and bake 25–30 minutes, or until a toothpick comes out clean.
03 - Remove cake from oven. Let cool for 10 minutes.
04 - Using the end of a wooden spoon, poke holes every 1–2 inches across the warm cake.
05 - In a large bowl, whisk together pudding mixes and cold milk until slightly thickened, about 2 minutes.
06 - Divide pudding evenly among 4 bowls. Tint each bowl with a different gel food coloring, mixing until vibrant.
07 - Spoon the colored puddings into the holes randomly to create a rainbow effect.
08 - Spread the whipped topping evenly over the cooled cake.
09 - Top with unicorn or colorful sprinkles.
10 - Chill in the refrigerator for at least 1 hour before slicing and serving.