Preheat the oven to 350 degrees Fahrenheit and spray 3 (8 inch) round cake pans with a non-stick cooking spray.

In a large bowl, beat with a hand held mixer or a stand up mixer the cake mixes, water, oil and eggs until the mixture is smooth and creamy.

Then divide the batter evenly into 6 small bowls (should be approximately 1 1/2 cups of batter in each bowl).  Use the gel food coloring to dye each bowl of icing a different color of the rainbow.  I used red, orange, yellow, green, blue and purple.

Refrigerate 3 colors of the batter and pour the remaining 3 colors each into a separate baking pan.

Bake for 18-20 minutes until a toothpick inserted into the middle of the cake comes out clean.

Let the cakes cool for 10 minutes and then carefully remove from the pans and allow them to cool completely on wire racks.

Wash the cake pans.  Re-spray for non-stick cooking spray and pour the remaining colors of the batters into the pans.  Bake the same as you baked the 1st 3 colors.  Let them cool completely.

In a large bowl cream the butter until smooth.

Beat in the confectioners sugar (one cup at a time) on low speed until well combined.

Stir in vanilla and milk.  Whip until thoroughly mixed and fluffy on a medium-high setting.

Use a serrated knife to trim the cake layers to make sure that both of the cakes are leveled.

Place the purple cake on a large plate or cake stand.

Spread a layer of the frosting on top of this cake.

Repeat this process with the blue, green, yellow, orange and red cakes.

Then frosting the sides of the cake and decorate the top as desired with the rainbow sprinkles.

Then the cake is ready to slice, serve and enjoy!