Divide your buttercream among three bowls and successively apply slightly more food coloring to each bowl. Load up three piping bags. After your crumb coat, pipe each color over about a third of the cake, from dark to light. At each spot where one color transitions into the next, use the tip of a spatula to swipe from the light into the dark, about an inch. Make little swipes around the the circumference of the cake. This helps the transition look less "perfect" and more natural. Use a cake scraper to scrape the sides of the cake smooth! If the transitions still look too stark, then repeat the little swipes with the spatula and scrape smooth again.