Streaks of buttercream frosting tinted with plant-based food dyes look like paintbrush strokes in this striking cake that feeds a crowd.
Preheat oven to 350 degrees. Lightly butter a 9-by-13-inch baking pan; dust with flour. Whisk together flour, baking powder, baking soda, and salt.
Beat butter with 1 cup sugar on medium speed in a mixer fitted with the paddle attachment until light and fluffy, 5 minutes. Add eggs, one at a time, beating until combined after each addition and scraping down sides of bowl as needed. Beat in vanilla. Reduce speed to low and add flour mixture in two batches, alternating with buttermilk and beginning and ending with flour.
Transfer batter to prepared pan, smoothing top with an offset spatula. Bake, rotating pan halfway through, until cake is golden and puffed and a tester inserted in center comes out clean, 22 to 25 minutes. Let cake cool completely in pan on a wire rack.
Frost top of cake with 2 1/2 cups buttercream. Divide remaining buttercream among three small bowls (about 1/2 cup each). For pink, add 1/4 teaspoon Berry food color. For red, add 1/4 teaspoon Berry and 1/8 teaspoon Sunflower. For yellow, add 1/4 teaspoon Sunflower. Let stand 10 minutes before using to allow dyes to fully saturate.
Using a small offset spatula, randomly scatter a few dollops of colored frosting around the cake (more white space will create a lighter effect, closer together will be more pigmented). Lightly drag a large offset spatula or bench scraper end-to-end across the cake to create a blended effect. Add more frosting as desired. Be sure to wipe off the spatula or bench scraper completely between strokes.