Line the cups up on a baking sheet that will fit all of them. No lids.
Place the yellow jello mix in a medium mixing bowl and add ½ cup boiling water. Mix until no graininess remains.
Add ¾ cup vodka and ½ cup cold water to the bowl. Mix well.
Using the injector, fill up each shot cup a third of the way.
Place in the fridge for an hour or until solidified.
Repeat steps 2-4, but use the grape jello instead.
Once the purple jello has solidified, repeat steps 2-4 for the pink jello.
Once the last layer has solidified, top with some whipped cream, if desired, and enjoy!