Preheat oven to 350F degrees. Spray a 9x13-inch baking dish with nonstick spray.
In a medium bowl, combine 1 box vanilla cake mix with 3.4 ounce box instant pistachio pudding. Mix those together until combined.
Then add in 3 large eggs, 1 cup oil and 1 cup cold milk. Stir just until combined.
Stir in about 6 to 8 drops of green food coloring until fully incorporated.
Pour into prepared baking dish.
Bake for about 30 minutes (see my not below, oven times may vary.) If you insert a toothpick into it, it should come out clean when done.
Allow cake to cool completely before making frosting.
To make frosting:
Pour 3.4 ounce box instant pistachio pudding into a mixing bowl. Add in 1/2 cup milk. Stir until combined.
Stir in 8 ounce tub frozen whipped topping, thawed.
Put in a few drops of green food coloring to really bring out that green color. About 5 drops or so.
Spread frosting onto cooled cake.
Slice and serve. If you like, sprinkle with some crushed pistachios.
Video
Notes
Please note: As with most baked goods, oven times can vary. Many factors play into oven baking. So you need to go by doneness not necessarily time. Use the time above as a starting point and go from there. In addition, cake mixes keep changing sizes. They used to be 18 ounces and they just continue to keep dropping in sizes. This recipe still works, but bear in mind, as manufacturers keep adjusting the sizing, it can also affect bake times and this recipe will not be necessarily updated each time to reflect possible change in baking times.