Rainbow swirl cake

My mum and I made this for my sister's birthday and used neon food coloring.

Rainbow cake:
Ingredients:
- any white cake recipe or use a box mix
- food coloring
 
Prepare your cake batter. Divide the batter into separate bowls and add food coloring to create the colors you'd like. Grease your cake pan(s) and line them with baking paper. Drop large spoonfuls of batter into the pans, alternating colors (there's a good image here of what it should look like). Don't stir. Bake for however long the recipe calls for and let the cake(s) cool before icing.

Buttercream: 
Recipe from: Six Sister's Stuff
 
- 1 c.  butter
- 1/2 c. milk
- 6-7 cups icing sugar

Beat the ingredients together. If it's too thin to spread, add more icing sugar, and if it's too thick, add more milk. Assemble and ice your cake.