For frosting, beat shortening, butter, reserved 3/4 teaspoon grapefruit zest and reserved 1/4 teaspoon lime zest in large bowl with electric mixer until smooth. Gradually add sugar, one cup at a time, beating well after each addition. Scrape sides and bottom of bowl often. Gradually add tequila, reserved 2 tablespoons grapefruit juice, reserved 1 tablespoon lime juice and icing colors; beat at medium speed until light and fluffy. Pipe or spread icing onto cooled cupcakes. Sprinkle cupcakes with coarse salt.