Place the plastic cups onto a baking sheet and set aside. Bring 2 cups of water to a boil.
Remove the water from the heat and whisk both packages of lime Jello powder into the hot water. Stir until the Jello is completely dissolved.
Add 1 cup cold water and 1 cup of vodka to the Jello mixture. Stir to combine.
Pour a shallow layer of lime Jello into the cups, filling each about a quarter of the way full. Cover the remaining lime Jello with plastic wrap and set it aside. Place the cups in the refrigerator to chill for 1 hour.
After the lime Jello has set, bring another 2 cups of water to a boil, then remove the boiling water from the heat. Whisk the in grape Jello powder and stir until dissolved.
Add 1 cup of cold water and 1 cup vodka to the grape Jello mixture. Stir until well combined.
Allow the mixture to cool completely, then add a layer of the grape Jello to the cups on top of the lime Jello. The cups should be about half full.
Cover the remaining grape Jello with plastic wrap and set aside. Place the cups back in the refrigerator to chill for another hour.
After the grape layer has set, take the remaining lime Jello from earlier, and add another layer, filling the cups about ¾ full.
Toss any leftover lime Jello (or set aside to eat as dessert later) and move the cups to the refrigerator to chill for 1 hour.
Once the second lime layer has set, add another layer of grape, filling the cups the rest of the way. Move them to the refrigerator to chill for 1 more hour, or until ready to serve.
Before serving, top with whipped cream and Halloween sprinkles.