Heat the oven to 350°F. Coat a 9×13 inch baking dish with butter or cooking spray, covering the bottom and sides completely to prevent sticking.

Whisk together the cake mix, vegetable oil, eggs, vanilla extract, and whole milk in a large bowl on medium speed for approximately 2 minutes until smooth—don't overmix. Transfer the batter to your prepared dish and bake for 25 minutes, checking doneness with a toothpick (it should come out clean or with a few moist crumbs). Let the cake cool at room temperature for about 30 minutes before continuing.

Meanwhile, pour the boiling water into four separate bowls, dividing it evenly so each bowl gets roughly ¾ cup. Add one gelatin flavor to each bowl, stirring for about 2 minutes until completely dissolved. Let the gelatin mixtures cool to room temperature, around 10 to 15 minutes.

Using the handle of a wooden spoon or a dowel, poke holes throughout the cooled cake in a grid layout, spacing them approximately 1 inch apart and going about ½ inch deep.

Once the gelatin has cooled, pour each color slowly into the holes in whatever pattern you prefer—alternating for stripes or creating distinct sections. Use about ¼ cup of each color, allowing each one to settle before adding the next. Any gelatin that spills onto the surface will set into a translucent layer. Cover the cake with plastic wrap and chill for at least 2 hours until the gelatin is firm and jiggles slightly.

Remove the chilled cake from the refrigerator. Spread the thawed whipped topping evenly over the top with a spatula. Add rainbow sprinkles if desired just before serving. Slice and serve cold, wiping your knife between cuts for clean, colorful slices.