Remove the cooled cakes from the pans. Have all of your buttercreams mixed, tinted and ready to go.
Set a cake board on a turntable. Set the chocolate cake on the cake board. Frost the top of the cake with approximately 1/2 cup of chocolate buttercream, then frost the sides with a thin crumb coat of chocolate buttercream.
Set the orange cake layer on top of the chocolate. Frost the top of the cake with approximately 1/2 cup of orange buttercream, then frost the sides with a thin crumb coat of orange buttercream, trying not to mix it into the chocolate buttercream on the layer beneath.
Set the yellow cake layer on top of the orange. Frost the top of the cake with approximately 1/2 cup of yellow buttercream, then frost the sides with a thin crumb coat of yellow buttercream, trying not to mix it into the orange buttercream on the layer beneath.
Set the white cake layer on top of the yellow. Frost the top of the cake with approximately 1/2 cup of white buttercream, then frost the sides with a thin crumb coat of white buttercream, trying not to mix it into the yellow buttercream on the layer beneath.
Place the crumb-coated cake in the refrigerator to chill for 30 minutes.
Fit four piping bags with a medium-sized round piping tip (such as Wilton #12). Fill the bags with the remainder of the chocolate, orange, yellow and white buttercream.
Set your chilled cake back on the turntable.
Starting at the bottom, pipe the chocolate buttercream around the cake in even rows, all the way around the cake, making sure each row touches the next. Continue piping around 1/4 of the way up the cake.
Repeat with the orange, then the yellow, and lastly the white buttercream.
Now, take a bench scraper or pastry scraper, hold it at about a 45-degree angle to the cake and lightly sweep it around the sides of the cake on a continuous motion – this is where it is extremely beneficial to decorate your cake on a turntable so that you can turn it with one hand while you smooth the buttercream with the other. Without a turntable, this step will be difficult for you. You are not trying to remove much frosting, so use a light hand – you are simply smoothing out any air bubbles and blending the colors together. Sweep around the cake several times, wiping the bench scraper clean with a paper towel each time around, until you’re satisfied with the result. Smooth out the top.
Make the ganache, then let it cool for several minutes until it's the right pourable consistency. You don't want it to be too warm so that it melts your buttercream, but too cool and it will thicken up to much to pour. Pour it on top of the cake, lightly spreading it towards the edges with an offset spatula and letting it drip down the sides.
Let the ganache set for about an hour, then decorate with candy.