Step 1: Preheat your oven and prepare the white cake mix in a 9x13 inch baking pan according to the package instructions. Allow the cake to cool for 15 minutes after baking.
Step 2: Using the end of a wooden spoon or a large straw, poke holes across the top of the cake, spaced about 1 inch apart and halfway deep into the cake.
Step 3: Divide the sweetened condensed milk into three small bowls. Add 1-2 drops of different gel food coloring to each bowl and stir until the color is uniform.
Step 4: Carefully drizzle each color of the milk over the holes in the cake, alternating colors to ensure a rainbow distribution.
Step 5: Cover the cake with plastic wrap and refrigerate for at least 4 hours, or overnight, to allow the milk to soak into the crumb.
Step 6: Spread the thawed whipped topping evenly over the top of the cake.
Step 7: Garnish generously with rainbow sprinkles and any other unicorn decorations before serving.