Beat butter and vanilla in large bowl with electric mixer on medium speed until light and fluffy. Gradually add confectioners' sugar, beating well after each addition and scraping sides and bottom of bowl frequently. Add milk; beat until light and fluffy.
Remove 2 cups of the frosting into medium microwavable bowl. Tint frosting orange using 1 teaspoon of the Sunflower color and 1/4 teaspoon Berry color. Set aside.
Tint remaining frosting green using 1 teaspoon Sky Blue color and remaining 1/2 teaspoon Sunflower color. Spread top of cupcakes with green frosting. Using a fork, gently touch frosting in different directions to resemble grass spikes.
To make the pumpkins, microwave the orange frosting on HIGH 10 to 20 seconds or until runny. Using a fork, dip marshmallow halves into frosting mixture, then place on top frosted cupcakes. Let stand until pumpkin frosting has dried. Press a tootsie roll half in center of each pumpkin for the stem. Decorate leaves and vines with remaining green frosting, if desired.