Heat oven to 350?F (325?F for dark or nonstick pans). Spray bottoms and sides of 2 (8-inch) round cake pans with cooking spray.

In a large bowl, beat 1 box cake mix and ingredients called for on box with electric mixer on medium speed for 2 minutes, scraping bowl occasionally. Divide evenly between pans.

Bake for 28 to 33 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes; remove from pans to cooling racks. Cool completely, about 1 hour. Meanwhile, repeat mixing and baking with the second box of cake mix to make 2 more layers.

Line a large rimmed pan with waxed paper. Pour Lucky Charms? cereal into the pan. Pick out all the marshmallows; set aside. Reserve cereal pieces for another use.

Flatten cake layers by trimming off round tops. In 2 layers, cut a small round out of the center using a 4-inch round cutter. In a small bowl, mix 1 1/2 cups of the marshmallows, the white vanilla baking chips, and 1/3 cup of the candy sprinkles; set aside.

To assemble the cake: Place 1 uncut cake layer on a serving plate; spread 1/3 cup frosting on top. Top with 1 layer with center removed; spread 1/3 cup frosting on top. Top with the second layer with center removed; spread 1/3 cup frosting on top.

Fill the cake with the marshmallow mixture. Gently press to level; top with the full cake layer. Frost the top and side of the cake with 1 1/2 cups frosting to form a crumb coat. Refrigerate for 30 minutes.

In a medium bowl, place 1 1/2 cups of the frosting; stir in 1/4 teaspoon blue food color until blended. Fit a decorating bag with a 1/2-inch star piping tip; fill with 3/4 cup frosting. Set decorating bag and frosting aside.

In 2 small bowls, place 1/4 cup each of frosting. To 1 bowl, add red food color to desired pink color; to the other bowl, add yellow food color to desired color.

Spread 1/2 cup of the blue frosting on top of the cake, reserving remaining frosting for the top third of the cake. Starting on the side, spread pink frosting on the bottom third of the cake. In the middle third of the cake, spread yellow frosting so yellow frosting overlaps pink frosting. On the upper third of the cake, spread remaining blue frosting so blue frosting overlaps yellow frosting.

Using an icing spatula, smooth the sides of the cake to blend colors. Wipe spatula clean with a paper towel as needed. Repeat blending of frosting and wiping off spatula. Smooth the top if needed. Pipe frosting rosettes around the top edge of the cake. Sprinkle remaining marshmallows and candy sprinkles in center.