Everyone will want a serving of this fruit salad: It's sitting in a bowl of cake! To make the faux watermelon, we dyed cake batter pinkish red and added chocolate chips to look like seeds. We baked it in a bowl to get the right shape, then we painted the icing with green and yellow food coloring to look like the rind.
Preheat the oven to 300 degrees F. Coat a 4-quart ovenproof bowl with cooking spray. Prepare the cake mixes as directed; fold in 2 teaspoons red food coloring and the chocolate chips.
Transfer the batter to the prepared bowl; bake until a skewer inserted into the center comes out clean, about 1 hour 30 minutes. Let cool slightly, then invert onto a rack to cool completely.
Put the cake flat-side down on a small cardboard circle. Trim 2 opposite sides with a serrated knife to make an oval shape.
Cover the sides of the cake with a thick layer of frosting; cover the top with a thin layer (this will be the bottom of the cake). Reserve about 1/2 cup frosting for the inside of the cake. Freeze until the frosting is firm, at least 20 minutes.
Gently pat the sides of the cake with a smooth paper towel to even out any bumps in the frosting.
Mix 1/2 teaspoon yellow food coloring with a drop of green food coloring and a splash of lemon juice in a small bowl. Paint the yellow-green food coloring all over the side of the cake with a pastry brush.
Mix 1/2 teaspoon green food coloring with a splash of lemon juice in another bowl. Paint green stripes on the cake, then blend the edges of the stripes with the tip of the pastry brush.
Put another small cardboard circle on the cake and flip it over. Carve out the center of the cake, about 1 inch deep, leaving a 1-inch border. Cover the center with frosting, leaving the border unfrosted. Freeze the cake 20 minutes.
Use a paring knife to make zigzag cuts around the rim of the cake. Mound the fruit in the center, covering the frosting completely.