Preheat the oven to 350°. Spray a 9 x 13 baking dish with nonstick spray. In a large bowl, combine cake mix, 1 box of instant pudding mix, eggs, soda, oil, and green food coloring and mix with electric mixer on medium speed for about 3 minutes. Pour into the prepared baking dish. Bake for about 25 -30 minutes or until inserted toothpick comes out clean to indicate the cake is done. Allow the cake to cool completely.
For the Frosting: In a bowl, and with an electric mixer, blend the butter until smooth. Add the vanilla, salt, food coloring and milk and continue to blend. Add the powdered sugar a little at a time and continue to whisk until frosting is light and fluffy and the consistency you would like.
Decorate the cupcake to look like a rainbow with a pot of gold at the bottom. Use skittles for the rainbow, marshmallows for the clouds, junior mint for the pot, gold sprinkles for the gold. You can add 4 green skittles as the shamrocks.