Rainbow Birthday Cake - A Decorating Tutorial

Here is a rainbow birthday cake I made for a customer recently. The theme was 'rainbow in the clouds', hence I designed this birthday cake for her.

A pretty pastel rainbow birthday cake

The cake itself was pretty simple, with a 2D rainbow on the side and lots of clouds at the base of the cake, the sides as well as on the top. And to add more rainbow colors to the birthday cake, I made the "HAPPY BIRTHDAY' letters in rainbow colors too!

How to Make a Rainbow Birthday Cake

Cake - This was a single tier cake, measuring 6 inches in diameter. The height of the cake was also 6 inches. Inside, the birthday cake consisted of 3 layers sandwiched with buttercream. I used my marble cake recipe for the cake. Since the recipe calculations on my recipe page were meant for cakes with a height of 4 inches, I multiplied the recipe by 1.5 times.
Buttercream - I used buttercream to fill in between the 3 cake layers as well as to crumb coat the cake. My buttercream recipe can be found here.
Fondant - I used Satin Ice fondant in white and baby blue. The baby blue fondant was to cover the birthday cake, while the white was for the clouds. I also used fondant for the rainbow letters on the cake as well as the 2D rainbow on the side of the cake.
Shortening and fondant glue - I used both these mediums as glue on the birthday cake.
Satin ribbon - I used a light blue satin ribbon which I purchased from a local craft shop to wrap around the cake board.

As in most of my cakes, I try to prepare decorations in advance so that I don't have to rush at the very last minute. For this birthday cake, I prepared the 3D fondant clouds and the fondant letters a few days in advance, not only because they could be made in advance but also because they needed time to set and harden so that they could be placed upright on the cake.
I have prepared 2 separate tutorial (detailed, step by step instructions) on how to make the fondant clouds and the rainbow letters. Click on each of the images below to learn more.

Closer to the delivery day, I baked the cake, sandwiched the layers with buttercream, crumb coated, and covered it in Satin Ice Baby Blue fondant. I also covered the cake board in the same color.
Next, I attached the 2D rainbow on the side of the cake. I used the same colors as the rainbow letters and used my extruder tool to pipe out consistent-sized strips for the rainbow. Click on the image below for a detailed tutorial on how to mix the rainbow colors and how to make the 2D fondant rainbow)

After the rainbow and the ribbon, I worked on the rainbow 'HAPPY BIRTHDAY' letters on the cake. The letters 'HAPPY' were positioned on the largest 3D cloud on the cake. The alphabets 'BIRTHDAY', on the other hand, were glued on the cake itself.
Before gluing the rainbow letters, I arranged them on the cloud and made sure they were centered. And then, I left the letters aside to dry completely.

Similarly for the word 'BIRTHDAY', to make sure they were perfectly centered, I arranged them in a lying down position as shown below. And then I adjusted the position until the letters were well centered.

Only then did I glue them one by one, starting from the middle, with fondant glue.

Rainbow alphabets and 2D rainbow

Next were the clouds. There were 2 types on this rainbow birthday cake. The first ones are 2D on the side of the cake. The other ones are 3D clouds on top of the cake (which I prepared in advance to allow sufficient drying time). Again, I have a separate tutorial for these 2D clouds. Click on the image below for the detailed instructions and photos (click here for the 3D clouds tutorial).

And finally, the 3D clouds went on. I started with the largest one and attached it to the cake with some fondant glue. I had a bit of difficulty in making these stay in place while drying and should have used a long wooden pick inserted to the bottom of the clouds so that they could be held in place more securely. But since the picks should have been inserted while fondant was still soft, it was kind of too late for the picks for me. Instead, I used my food coloring bottles to help support the pieces from falling off while the glue was drying.
Once dried, I accented them with clusters of fondant balls on the cake.