Preheat the oven to 325 degrees F. Grease two 7-by-7-inch cake pans.
In an electric mixer fitted with the paddle attachment, cream the butter and sugar for 2 minutes. Scrape down the sides of the bowl, and add the vanilla. Add one egg at a time, mixing between each addition. In another bowl, sift the cake flour, all-purpose flour, salt and baking powder. Mix together the milk, lemon juice and lemon zest. Set the mixer to the to the lowest speed, and alternate adding the flour mixture and the milk mixture a little bit at a time, starting and ending with the dry mixture. Scrape down the bowl to make sure all the ingredients are fully combined. Separate the batter evenly into three mixing bowls. Use the food coloring to color one bowl bright green, one bright orange and the last bright pink.
Scoop each of the colors randomly into each pan. Use a small knife or spatula to swirl the colors into the cake to create a marble effect. Bake until cooked through, 45 minutes.
Cool the cakes for 20 minutes.
Frost one of the cakes with Rainbow Sherbet Buttercream, then layer the other cake on top. The cake will look colorful and marbled when cut.
In an electric mixer fitted with a paddle attachment, cream the butter. Add the powdered sugar and whip until fully mixed, scraping the sides of the bowl as needed. Add the lemon and orange emulsions, then separate the buttercream into 3 equal parts. Color one green, one orange and the last pink. Put all three frosting colors in a pastry bag. When piped, the colors will blend together.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.