Beat butter, shortening and vanilla until creamy. Add powdered sugar, one cup at a time, beating at low speed until well blended. Add milk and marshmallow cream, beat until light and fluffy.
Set aside 1 cup of frosting divided into 2 small bowls. Color 1/2 cup of the frosting green and the other 1/2 cup brown.
Stir in orange food coloring into the remaining frosting until it is the shade of orange you want.
Put the orange frosting into a pastry bag with a large tip. Hold the bag directly above the cupcake and squeeze out the frosting to that is will spread out like a pumpkin.
Put the green frosting into a pastry bag and use a star shaped tip to create leaves on the pumpkins.
Put the brown frosting into a pastry bag and use a small tip to make pumpkin stems.
Sprinkle with orange sugar crystal sprinkles, if desired.