Mix the cake mix following the instructions on the box. Then add the mint.
Divide the cake mix equally between three bowls. Stir in 12 drops of green coloring to the first bowl, 6 drops to the second bowl, and 3 drops to the last bowl.
Spray the bottom of a 9 by 13 inch baking dish with cooking spray. Spread the darkest bowl of cake mix over the bottom. Put it in the freezer for 20 minutes.
Very carefully spread the second darkest layer over the first. Put it back in the freezer for 20 minutes.
Finally, spread the lightest layer of green very gently over the first two. Preheat your oven according to the box instructions. Set the pan on the top of the preheating oven for about 15 minutes. This will help bring all the layers to the same temperature so that they bake evenly.
Bake according to the instructions on the box.
Remove from the oven, wait about 10 minutes, and then poke holes throughout the top of the cake with the end of a wooden spoon. Pour the sweetened condensed milk over the cake, making sure some gets in each hole. Refrigerate overnight.
Top with cool whip and M&Ms and serve.