Spoon 1/2 cup vanilla frosting into a bowl. Spread a thin layer over the top of each cupcake with a knife. Refrigerate cupcakes for 30 minutes to make decorating easier.
Combine remaining 2 cups of vanilla frosting and green food coloring in a bowl; stir until frosting is uniformly green. Add more green food coloring, 1 drop at a time, until your frosting is the color you like.
Transfer frosting to a piping bag. Working perpendicular to the top of each cupcake, pipe the green frosting around the edge, working inwards in a circular motion to create a tree shape. Repeat with remaining cupcakes.
Decorate Christmas trees with sprinkles, colored sugar, or candies as the "ornaments" on the trees.