Fill and frost cupcakes just before serving if you want a crunchy marshmallow surprise. Or, fill them at least 2 hours ahead for a soft marshmallow center. Gold foil or rainbow cupcake liners make festive St. Patrick's Day cupcakes.
Heat the oven to 350 degrees F (325 degrees F for dark or nonstick pans). Line two cupcake pans with paper baking cups.
Make the cake batter as directed on the box. Divide the batter evenly among the muffin cups so that each is about two-thirds full. Bake about 20 minutes or until the cupcakes spring back when touched in the center. Cool the cupcakes in the pans 10 minutes and then transfer them to cooling racks to cool completely, about 30 minutes.
Using serrated knife, cut a divot out of center of each cupcake, about 1 inch deep. Reserve the cutout cake pieces. Fill each cupcake with 1 tablespoon of Lucky Charms marshmallows (about 10 marshmallow pieces) and a few rainbow sprinkles. Trim the reserved cake pieces flat, and cover the holes in the filled cupcakes.
Spoon the frosting into a medium bowl and stir in enough food color for desired shade of green. Transfer the frosting to decorating bag fitted with large star tip, and pipe the frosting onto the tops of the cupcakes. Or, frost the cupcakes with an offset spatula. Garnish with rainbow sprinkles.
Store cupcakes at room temperature loosely covered with plastic wrap.