Preheat the oven to 350 degrees and grease a small square cake pan.
In a mixing bowl combine the funfetti cake mix with the eggs, vegetable oil, and water. Stir until smooth.
Add a few drops of green food coloring to the batter and mix until the color looks even.
Pour the batter into the prepared pan and bake for 25 to 30 minutes until a toothpick comes out clean.
Let the cake cool completely before removing it from the pan.
In a separate bowl tint the vanilla frosting with green food coloring until it reaches the desired shade.
Spread the green frosting evenly over the top of the cooled cake.
Scatter the rainbow sprinkles across the frosting and press the candy monster eyes into place.