Bake the cupcakes according to package directions. Cool completely.
Scoop frosting into large bowl, add several drops of green gel food coloring. Stir to combine. Continue adding food coloring 1-2 drops at a time, mixing well between each addition, until frosting reaches desired shade.
Add frosting to piping bag, cut 1' off from the tip, then attach star decorating tip.
Starting in the center of each cupcake, squeeze the frosting out through the tip and move in a circular pattern working out to the edge of the cupcake and back in towards the center moving up to form a tree.
Decorate with sprinkles.