To get an ombré frosting effect, divide the frosting into individual bowls. You’ll want about two kinds of frosting for every layer of cake, so for a standard two-layer cake, you want four bowls of frosting. Add a bit more frosting to one of the bowls, which will be the color of the top layer (it should either be the lightest shade or the darkest shade). Mix food coloring into each of the bowls, increasing the amount of coloring for each bowl, so the colors get darker and darker. Frost the top of the cake with an offset spatula. Fill a piping bag fitted with a large, round tip with the lightest shade and pipe around the edge of the cake. If the lightest shade is on top, start at the top; if it’s not, start at the bottom. Once you’ve made a complete rotation, squeeze out all of the remaining frosting from the bag and then fill it with the next shade and pipe directly next to the frosting that you just piped on. Repeat until the whole cake is covered. Use a bench scraper to scrape off all of the excess frosting and create smooth edges (see step 3). Scrape off the bench scraper each time you scrape down the cake.