Make sure your cake has been correctly crumb coated and chilled. You will also want to place it over some parchment paper so that if you spill some frosting, it will not go over the beautiful stand.
Measure the cake and make sure the acetate sheet you have is big enough to cover the entire surface of the cake. If it is not, then you can always tape two together to make it as tall and big as you need it.
Grab your plain buttercream and divide it into four batches. You will want to use food coloring in growing intervals, say 1 drop in the second bowl, 2 in the third and 3 in the last one. You should go from pure white to intense color.
Place the acetate sheet on your working surface (be careful to place the tape toward the table if you had to tape two sheets together). And then, the fun part begins!
Using the frosting, cover the whole surface area of the acetate (leaving some space on the sides for you to hold onto it and place it afterward). Place the frosting with an offset spatula for a better effect.
You can also mix -but not too much!- some of the shades to make it look like marble swirls. You should do this in a bowl before applying the mixture onto the acetate.
Grab the acetate by the sides you left without buttercream and wrap it around the crumb-coated cake.
To do this, you should start in the middle of the acetate and work your way through to the sides, wrapping it as you go with each end. If you measured correctly, the acetate should be able to cover the entire length of the cake.
Smooth it with your hands or a bench scraper to make sure the whole of the icing is touching the cake. If not it will not stick correctly.
Place your cake in the freezer for 20 minutes. This will allow the buttercream to harden so that you can scrape off the acetate.
Scrape off the acetate carefully. Once you are done, check if there are any holes in the cake. Make sure to patch them up using some extra buttercream. Use a hot spatula or bench scraper while the frosting is still cool to smooth everything out.
Afterward, you will want to trim away excess frosting that has gathered at the top of the cake and clean the underside if needed.
You can now decorate the top of the cake however you like it! One of our favorites is piping a french tip border all around in either white or the full-on color you chose for your marble effect. It looks so pretty and elegant!
You can also drizzle some ganache and add some sprinkles. Go nuts!