Tint buttercream green with gel-paste food color. Transfer to a pastry bag fitted with a small multi-opening tip (#233). Starting at the center and moving outward, pipe rows of grass on top of cupcakes: With tip on surface of cupcake, squeeze bag, then release while quickly pulling up to form 1/4-inch-long blades, making some slightly longer than others.
Form ladybugs: Divide marzipan in half. Working on a surface lightly dusted with cornstarch, tint one portion red and the other black with gel-paste food colors. Form a 1/2-inch ball of red marzipan for each ladybug body, and a 1/4-inch ball of black marzipan for each head. Flatten the red ball slightly into an oval shape, and press the head onto the body. Pinch off black marzipan to make tiny balls for the spots, and press onto the body. Repeat to make additional ladybugs with remaining marzipan. Place one or two ladybugs on top of each frosted cupcake, gently pressing them into the buttercream grass. Refrigerate 30 minutes to allow frosting to set. Cupcakes are best served the same day they are decorated; keep at room temperature.