I came up with these ideas while making a rainbow cake for a little girl's birthday party that really wanted a rainbow cake. They may look hard but it is somewhat easy if you have a little cake decorating experience, it is time consuming to dye the batter though.

Needed for cupcakes/cake
Favorite white cake recipe or a boxed white cake mix- with ingredients to make it
Food coloring

Needed for frosting/top for cupcake
Heavy Whipping Cream
Powder Sugar
Food Coloring
Airhead Extreme Sours

Needed for frosting/top of cake
Two cans of vanilla frosting
Powder Sugar
Package of Skittles and berry Skittles
Food Coloring
Cake decorating star tip and bag

Step 1: Making Batter

Make the cake batter and seperate it into six small bowls.
Dye each bowl a different color- Red, Orange, Yellow, Green, Blue, and Violet
For cupcakes- take one spoon full of each color and layer them in the cupcake paper. If you are doing them in order VBGYOR
For cake- you will do three cakes. First 9 x 13 cake- Red, Orange, Yellow Second 9 x 13 cake- Green, Blue, Violet Third is a bundt cake- Red, Orange, Yellow, Green, Blue, and Violet. Try not to mix the colors, and carefully spread each color evenly in the pan.
Bake according to recipe

Step 2: Decorating

Cupcakes- After the cupcakes are baked and cooled make the frosting. Mix the heavy whipping cream with a blender until it thickens, then mix in powder sugar to give it a thicker texture and make it sweet. Then color half of it blue to look the sky, and the other half leave white for the clouds. Coat each cupcake with the blue, then cut each airhead extreme in half and place it on top in the shape of a rainbow, then put white frosting on the edges to help hold it in place and look like clouds.

Cake- the cake is made of two 9 x 13 inch pans, and one bundt cake pan cooked with the colored batter
When the cakes cool, stack the two 9 x 13 cakes, and then cut the bundt cake in half to make the arch of the rainbow.
Mix the cans of frosting with some powder sugar to make it more stable. Then divide the frosting and dye it to the colors needed.
Then using a star tip decorate using colored icing, and lining the edge with different colors, and skittles.