For frosting, in large mixing bowl, beat shortening and butter with electric mixer using the paddle attachment until light and fluffy for about 1 minute. Beat in mint extract.
Gradually add sugar, starting by mixing on low speed then beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been added, frosting will appear dry. Add milk; beat at medium speed until light and fluffy. Tint icing to desired shade of green.