Prepare the cake as directed. Before pouring into the pans, dye the cake the color of the fruit you're making (orange for the orange slice, red for watermelon, etc.) Pour into pans and bake as directed. We baked ours in smaller, 6" pans instead of the typical 8" or 9" but you can do any size! Let cool and frost according to the fruit you picked below.
For the citrus: Dye the majority of your frosting bright orange, and the rest a very pale orange. Frost the cake with the bright orange. Use a toothpick to draw the six wedges on the top of your frosting. Use a knife or piping bag to fill in the wedges you drew with the pale orange frosting.
For the kiwi: Dye the majority of your frosting a bright kiwi green (try green, plus a little yellow), and the rest a lighter kiwi green. Frost the cake with the brighter green. Frost a circle in the center with the lighter green. Pipe black seeds all along the outside of the circle.
For the dragon fruit: Dye the majority of your frosting fuchsia, and the rest you can leave white or add a touch of purple to make it a grayish color. Frost your cake with the hot pink. Frost a circle, covering most of the top of the cake, with the white/gray. Pipe black seeds all over the circle.
For the watermelon: Dye the majority of your frosting two shades of green (one lighter, one darker), and the rest a bright red. Frost the cake with the darker green. Use an offset spatula to mix in some of the lighter green, making blurred "stripes" like a watermelon would have. Frost a circle, covering most of the top of the cake, with the red. Pipe black seeds all over the circle.