To prepare cake: Prepare cake layers and frosting as directed. Spread ¾ cup frosting between each layer. Spread remaining frosting in a thin layer (crumb coat) to top and sides of cake making sure surfaces are very smooth, filling in any wholes or gaps with frosting. Reserve 1/3 of fondant (about 8 ounces); cover and set aside. Microwave remaining fondant on HIGH in 30 second intervals until softened and pliable. Knead in coloring (orange and golden yellow) until desired color is achieved. Roll out on a smooth surface dusted lightly with cornstarch to ¼ inch thickness. Follow package directions for applying fondant to cake.
To make the orange slice decoration: Microwave reserved fondant on HIGH in 30 second intervals until softened and pliable. Roll out on a smooth surface dusted lightly with cornstarch to ¼ inch thickness. Using the top of an 8-inch cake pan as a guide, cut out an 8 ½-inch circle. Reserve fondant scraps. Press down on outside edge of circle to smooth. Dampen the top of the cake with a wet paper towel. Center white fondant circle on top of cake, pressing gently with fondant smoother. To create orange section guides: Knead together fondant scraps and roll out a 9-inch long strip (1/8-inch thick). Cut four (1/4-inch) strips and roll between fingers to round the edges. Dampen and lay across top of cake in a spoke like fashion to create 8 wedge shaped sections. Dilute a few dabs of icing color (orange and golden yellow) with 1 tablespoon water. Test color with a small paint brush on a fondant scrap and adjust color as needed. Paint orange sections starting from the inside working outward, leaving a ¼-inch border around outside edge. Use short strokes to resemble orange pulp. Carefully remove fondant guides and discard.
To make orange seeds: Pinch off eight (¼-inch) balls from white fondant scrap. Flatten and elongate one side to create a tear drop shape. Place one seed in each orange section.
To make orange leaves: Tint a small piece of scrap white fondant with leaf green coloring. Roll to 1/8-inch thickness. Cut out a leaf shape. Score the leaf with the back of a small knife to resemble veins. Arrange on side of cake.