Combine the butter and shortening in a mixing bowl. Beat on medium speed with an electric mixer until creamy. Add the vanilla and salt. With the mixer running, add the confectioners' sugar in 1 cup increments, mixing well after each addition. Add the milk and food coloring and beat on medium-high speed until fluffy, scraping down the sides of the bowl as needed. Cover the bowl and refrigerate for 30 minutes before using. To use as a frosting, thin the icing with milk until it is a spreadable consistency. For pure white icing, use all shortening and omit the butter and food coloring paste.