Preheat the oven to the temperature specified on the cake mix box. Grease a 9×13-inch baking dish with non-stick spray and set aside.

Prepare the cake batter according to the package instructions. Divide the batter evenly into four separate bowls. Add a few drops of pink food coloring to the first bowl, blue to the second, green to the third, and yellow to the fourth. Stir each until the color is well combined.

Using a spoon, drop dollops of each colored batter randomly into the prepared baking dish, layering and alternating colors to create a marbled effect. Gently tap the dish on the counter to remove air bubbles and slightly swirl the colors together with a toothpick, being careful not to overmix.

Bake the cake according to the time recommended on the box, or until a toothpick inserted into the center comes out clean. Remove from the oven and let it cool for about 10 minutes.

With the handle of a wooden spoon or a large straw, poke holes approximately 1 inch apart all over the surface of the cake. Allow the cake to cool completely.

In a medium bowl, whisk together the white chocolate pudding mixes and milk until the pudding begins to thicken but is still pourable. Pour the pudding over the cake, spreading it evenly and ensuring it fills the holes. Refrigerate the cake for about 30 minutes to allow the pudding to set.

In a large mixing bowl, combine the heavy cream and vanilla extract. Using a hand mixer, beat until soft peaks form. Add the powdered sugar and continue to beat until stiff peaks form. Spread the whipped cream over the set pudding layer on the cake.

Decorate the top with pastel sprinkles and arrange the bunny-shaped marshmallow Peeps as desired.