Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13 inch baking pan. In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla. In a separate bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt. Add dry ingredients to wet ingredients alternately with buttermilk, beating until just combined. Stir in food coloring until desired color is achieved. Pour batter into prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely before frosting.