Heat oven to 350°F (325°F for dark or nonstick pan). Grease 15x10x1-inch pan and line bottom of pan with waxed paper. Lightly grease waxed paper; lightly flour. Or spray pan with baking spray with flour.
In large bowl, make cake batter as directed on box. Pour into pan. Bake 21 to 27 minutes or until toothpick inserted in center comes out clean. Cool cake 5 minutes; remove from pan to cooling rack. Peel off waxed paper. Cool completely, about 45 minutes. Freeze cake about 30 minutes or until very firm.
Place cake on cutting board. Cut roof line and around silo (as shown in diagram). Cut 17x12-inch piece of sturdy cardboard; cover with heavy-duty foil. Carefully transfer cake to foil-covered cardboard.
Stir red icing color into 1 container of the frosting to make desired red color. Frost barn and silo--except for upper rounded section--with red frosting. Make vertical lines over cake with rubber spatula to look like boards.
Place 2 crackers side by side on barn for doors. Place 1 cracker at an angle under peak of barn for window; press into frosting. Arrange pretzel sticks around window and door; make X on each door with remaining pretzel sticks.
Spoon 1/2 cup white frosting into decorating bag fitted with small star tip. Pipe white frosting along edge of roof, and fill in top rounded section of silo.
Shake coconut and green food color in tightly covered container until coconut is evenly tinted. Arrange coconut along bottom edge of cake for grass. Arrange shoestring potatoes in window and at bottom of cake for hay. Place animal crackers on cake above grass. Store loosely covered.