Sift together the flour, baking soda, salt, and cocoa. Mix the sugar and eggs. Add the oil, slowly beating well as the oil is added. Add the flour mixture alternately with the buttermilk, beginning and ending with the flour and mixing well after each addition. Stir in the vanilla and food coloring. Divide the batter evenly among the pans and bake for 40 minutes, testing for doneness with a toothpick. Cool the layers in the pans for 10 minutes, then turn out onto racks to finish cooling while you prepare the frosting.