Place the pink cereal into one bowl and pour on 1 cup of milk; stir until fully soaked. Place the blue cereal into another bowl along with the remaining 1 cup of milk; stir until fully soaked. Let both cereals steep for 30-40 minutes.

Place a fine-mesh sieve over a medium bowl and strain out the milk from the pink cereal. Discard the soaked cereal and stir in 1 tablespoon of sugar. Repeat with the blue cereal and remaining tablespoon of sugar in another bowl.

Preheat oven to 350ºF degrees. Line three 6-inch round cake pans with parchment paper and spray with nonstick cooking spray.

In a food processor (or with a ziplock bag and rolling pin), finely crush the pink cereal, and empty into a small bowl. Repeat with the blue cereal, placing crushed blue cereal into another small bowl. Set crushed cereals aside.

In a large mixing bowl, whisk together the flour, baking soda, baking powder, salt, and sugar until combined. Add the oil, yogurt, egg, and vanilla stirring until combined. Batter will seem thick and doughy.

Divide the batter equally into two separate mixing bowls. To one bowl of batter, add the pink cereal milk and pink crushed cereal. Stir until just combined. In the other bowl of batter, add the blue cereal milk and blue crushed cereal, stirring until just combined.

Divide both batters equally among the three prepared pans by alternating between spoonfuls of blue batter and spoonfuls of pink batter. Bake in the oven for 28-32 minutes, or until a toothpick inserted in the center comes out clean. Allow cakes to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and salt on high until fluffy, about 3 minutes. Add the powdered sugar, a cup at a time, until fully incorporated. Mix on high for 3 minutes. Add the vanilla, followed by the heavy cream, a tablespoon at a time, beating until fluffy, about 2 minutes.

Remove 1 cup of the buttercream and set aside for the filling. Transfer the rest of the buttercream to a separate bowl, then return bowl to mixer.

In the mixer, beat the cream cheese and reserved 1 cup of buttercream until smooth and combined. Add half of the whipped cream, and continue beating until combined. Remove bowl from mixer, and fold in the rest of the whipped cream.

Place a cake layer on a cake plate or pedestal. Spread 1/3 of the filling on top, and repeat with the remaining cake layers and filling, ending with a layer of filling on the top. Chill cake in the freezer for 1 hour.

Once chilled and solid, frost the entire cake with the buttercream. Refrigerate cake for at least 30 minutes to allow buttercream layer to set up. While the cake is chilling, divide the remaining buttercream among three bowls.* Tint one bowl of buttercream to the desired shade of pink, and another bowl to the desired shade of blue.

Remove cake from the fridge, and use an offset spatula to spread thick patches of pink and blue buttercream all around the cake. Fill some of the areas with some of the white untinted buttercream.

Smooth the buttercream around the cake as the colors begin to blend together. If there are empty patches, feel free to go over them with more buttercream until you get the desired watercolor/marble effect you’re looking for.**

Transfer the remaining white buttercream to a pastry bag fitted with a star tip (I used Wilton's 1M), and pipe a border of stars around the top of the cake. Add sprinkles, as desired. Refrigerate cake until ready to serve.