For Cake: Make a paste of red food coloring and cocoa. Cream shortening and sugar thoroughly. Add eggs and mix well, then add the paste. Add buttermilk alternately with the flour and salt. Beat well after each addition and then add vanilla. Put soda into the vinegar and fold into the batter. Pour batter into 2 greased 9-inch cake pans and bake in a 350 degree oven for 25 to 30 minutes. Cool and split each layer in half, making 4 layers. Frost layers with icing.
For Frosting: Cook flour and milk until thick. Then cool. Be sure it is cool. Cream together margarine and sugar. Add vanilla. Beat the two mixtures together until the consistency is that of whipped cream.