Mix cake according to manufacturer’s instructions. Measure out 4 pounds batter, deposit 1 pound into a sprayed and papered 8-inch cake pan. Place 2 pounds of batter in a bowl, the remaining batter in a different bowl. Color the 2 pounds of batter with pink food coloring then deposit 1 pound each into sprayed and papered 8-inch cake pans. Color the remaining 1 pound of batter with green food coloring and deposit into a papered 8-inch cake pan. Bake at 350F until cake springs back lightly when touched. Let cool completely.
Once cooled, trim the bellies of each cake, saving them for other uses. Then split the white layer in half, reserving on half for another use.
Place buttercream into 4 bowls, flavor each with watermelon flavoring. Color one bowl pink, leave one white, color another light green, and the last bowl dark green. Place the dark green cake layer on a cake board, top with green buttercream, spread to edges. Place a white layer on top and top with pink buttercream, spread to edges, top with a pink cake layer followed by pink buttercream, then another pink layer. To crumb coat the cake, place each color of buttercream into a piping bag and pipe corresponding colors around the cake in circles. Scrape off excess buttercream. Place internal supports/dowels into the cake. To smooth ice the cake, pipe corresponding colors around the cake in circles and smooth ice using a heated spatula if needed. Decorate with candy watermelon seeds.