Sort cereal so that each color is in its own bowl. You should have about 3 cups of each color (6 colors total).

Line the bottom of three 8-inch round cake pans with parchment paper circles cut to fit the bottom of each pan. Spray each pan with non-stick cooking spray. Cut three additional parchment paper circles to use in between layers of cereal. (If you only have one or two cake pans, you will just have to repeat the process once the layers in your pan(s) have cooled.)

In a large microwave safe bowl, melt 1-1/2 tablespoons butter with 3 cups mini marshmallows for 1 minute. Stir until marshmallows are melted and smooth. Add 3 cups of one of the colors of cereal. Gently stir until cereal is well coated in the melted marshmallows. Spray hands or a piece of parchment paper with non-stick cooking spray and press cereal evenly into one of the prepared cake pans.

Place a circle of parchment paper, that's been sprayed with non-stick cooking spray, on top of the cereal you just pressed into the cake pan. You'll layer another layer on top of this layer to maximize cake pan space.

Repeat process with remaining cereal colors, marshmallows and butter, melting 1-1/2 tablespoons butter and 3 cups mini marshmallows at a time for each color of cereal. You can use the same bowl for melting the marshmallows and stirring in the cereal.

Press each color into the prepared cake pans with parchment paper on the bottom and in between each layer. Allow cereal layers to cool. To speed this process you can place them in the refrigerator for 10 minutes.

Once cereal layers have cooled, turn pans upside down to allow cereal layers to fall out. If the cereal has stuck to the side of the pan, gently run a table knife around the edges before inverting.

Begin layering the rainbow cake on a cake stand or platter in the following order, from bottom to top: purple, blue, green, yellow, orange, red. Be sure to remove the parchment paper as each layer should set directly on top of the other. Gently press down on the top of the cake so layers press into each other. Place cake in the refrigerator for about 15 minutes to let it set before slicing and serving.