Reserve about 3/4 cup white buttercream in a pastry bag fitted with a large plain round tip. Color about 1 1/2 cups buttercream deep green. Color about 3 cups buttercream deep pink, with a few drops of red added to make the color of ripe watermelon.
Arrange 19 cupcakes in a triangle shape, starting with 1 at the top and ending with 6 at the bottom, sloping the bottom sides up to resemble the curved rind of a watermelon wedge.
Use an ice cream scoop or large spoon to dollop about 1/4 cup green buttercream on each of the 6 cupcakes in the bottom row. Smooth with a small offset spatula, covering the tops and sides (but not the paper liners), to make a solid line of green for the rind.
Wash and dry the ice cream scoop and offset spatula. Dollop about 1/4 cup pink buttercream on each of the remaining cupcakes and smooth the tops and sides with the offset spatula. Shape the frosting on the top cupcake into a point to resemble the tip of a watermelon wedge.
Wash the offset spatula again. Pipe a curved line of white buttercream along where the pink and green buttercream meet. Smooth with the tip of the offset spatula to make the inner white part of the watermelon rind.
Press jelly beans in a random pattern to make watermelon seeds.
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