Here are 3 easy rainbow cake designs that are quick to decorate but stunning to look at!
#1 Piped rainbow cake
For this first easy rainbow cake design I’m using sprinkled cake layers for an extra burst of colour. This is my Very Vanilla cake with pink nonpareil sprinkles, called hundreds and thousands in the UK. You could use rainbow sprinkles to make the cake rainbow coloured inside and out!
After assembling your cake, cover it with a thin layer of frosting called a crumb coat. This doesn’t need to be perfect because it’s going to be covered up in a minute. However, getting it nice and smooth will make the rest of the decorating easier and neater. Let the frosting set while you prepare your colours.
After the crumb coat sets, spread more frosting onto the top of the cake and smooth it. The top of the cake is going to be visible at the end so you want it to be neat. After smoothing the frosting on top, use a cake comb to flatten it around the sides.
How to pipe rainbow colours onto the cake
For the most eye-catching results choose several different piping tips. My favorites are star tips like a 1M, #32, #4B, #199 and petal tips like a #104 or #125.
Choose a colour to start with. This is a #32 open star tip with my 4 Minute Buttercream tinted pink with gel colours. To pipe a shell border like this, hold the piping bag on its side and squeeze the buttercream out. Hold still for a moment to let the buttercream bulge out. Then pull away to leave a tail on the dot. Overlap each dot all around the cake to pipe a row of these shells or waves.
Where you start and finish there will be a join that looks different to the rest of the texture. If you start above that with your next colour, all of the joins line up over each other. This can be the back of your cake.
This is a 1M star tip and I’m using the same technique to pipe a shell border with it. This piping tip is much bigger than the others so the piped details are bigger, too. It looks really big now but once the rest of the details are added, I think the variation looks nice.
Sometimes it’s useful to have a guideline to mark the top of the row of piping. You can use the straight edge of a cake comb to indent one into the crumb coat.
To pipe ruffles with a petal tip, hold the tip with the narrow end of the teardrop shape pointing outwards and the wide end pressed against the cake. Squeeze the bag as you move it slowly up and down around the cake. Pause for a split second at the top and bottom to allow the buttercream to curve softly around.
The bottom of the piping tip should touch the piping underneath it every time you go down. That way you’ll make sure you cover up the crumb coat completely with no gaps in between the different colours of piping.
There’s no “right” way to create the join between the beginning and end of piping. Create whatever texture you can and as long as you’re lining up your joins above each other, that side can face the wall when you display the cake so no one will see it anyway!
Couplers are useful if you want to be able to use the same piping tip with different colours. After piping with one colour, unscrew the ring to remove the piping tip and then place it on another coupler on another piping bag. Squeeze the bag to push out the previous colour until you see the new colour come through.
Being able to switch between piping tips by using couplers really saves a lot of time when you’re piping. And by re-using piping tips with different colours, you’re limiting the number of tools you need to buy. You're also reducing clean up afterwards! Since the same texture on the cake has different colours, it still adds lots of variety in your design.
I’m using pale rainbow colours but you can use bright colours instead. To make darker buttercream you’ll need to add a few more drops of colour when you tint your frosting. I’m using my 4 Minute Buttercream which takes on colour very easily so with just a few drops you can create really bright and bold colours.
Round tips like this 2A pipe dots or beads. The bigger the tip, the bigger the beads. Use the same shell border technique to pipe beads with tails and overlap each one.
When you get to the top of the cake and you’re tempted to stop, keep in mind that the cake will look neatest if your final row of piping sticks up above the top edge of the cake.
Voila! Beautiful colours and textures that make a quick and easy but stunning cake design!
#2 Rainbow layer cake
Skip the texture and create flat layers of your rainbow colours around a cake with this next technique.
You can either spread or pipe the colours onto the cake. Since I already have my colours in piping bags I'm piping them on but if you don't, it's quicker to spread them on. Start at the top of the cake, creating a ring of colour all around it. Then scrape around the cake gently with a cake comb to leave a thin, smooth layer of colour on the cake.
After each time you go around the cake, scrape the excess frosting off your cake comb into a bowl. If you take off more frosting than you want you might be able to see the frosting on the cake through the band of colour. Spread some of the scraped-off coloured frosting back onto the cake and then smooth it again.
Pipe or spread your next colour underneath, overlapping the first colour so that there’s no gap in between the two colours.
Three tricks to this rainbow layer cake design
First, use a crusting buttercream like my 4 Minute Buttercream. It gets really firm when it sets and makes this layering effect possible. With soft frostings like whipped cream, the colours blend into the frosting on the cake instead.
Second, take your time to get the frosting really smooth. These colourful layers won’t completely cover the cake and they won’t disguise messy frosting.
Third, chill the cake in the fridge for at least two hours before you do this. That way, the frosting will be very firm and you can pipe or spread and scrape the colours without damaging it. Also, the colours will set very quickly when they’re on top of very cold frosting so the colours will create layers over each other instead of blending together.
Make a rainbow on top of the cake
To add some detail to the top of this cake, squeeze a little bit of each colour of frosting onto the straight edge of a cake comb. My cake is tiny, it’s only 4 inches wide, so I’m using the bottom edge of the comb instead of the side.
Then lower the comb onto the top of the cake and pull it in a curved, arc shape over the top surface of the cake to leave a rainbow behind! You might need to spread on a bit more of some of the colours and scrape again to get good coverage.
You can use a piping tip or a toothpick to draw a design in the rainbow as well, if you like!
#3 Rainbow Tray Bake
If you want rainbow cake that’s very quick to decorate, this one’s for you! Pour your cake batter into a baking dish or casserole dish and bake it in that. Let it cool completely and then decorate your cake in the dish, so you don’t need to assemble and frost it first!
You can spread frosting onto the cake or use piping tips to create colourful texture like on the first cake. Piping down onto this cake is easier and quicker than piping onto the side of a cake!
The best bit about this rainbow tray bake cake is that transporting and serving it in a dish is so simple!