Place the gelatin in a medium mixing bowl. Pour in the cold water and stir to combine; set aside.
Heat the pineapple juice in a small saucepan over medium-high heat and bring to a simmer.
Once the gelatin has absorbed into the cold water, pour in the hot juice and whisk to combine until the gelatin dissolves. Add the sugar and whisk to dissolve.
Pour in the rum and 1 drop of green food color, stir to combine. If you're not happy with the color of the shots, you can add one more drop of food color at a time until your desired color is reached.
Removed the plunger from each syringe and place them in a cup or something that will keep them vertical, but be sure to leave the cap over the tip of the syringe.
Pour just over 2 ounces of the liquid into each of the syringes (so they're almost full). Leave the plunger off the shots until they set. Put the cups in the fridge and chill until the jello starts to set, about 1-2 hours.
Remove the shots from the fridge and take the caps of the tips. Insert the plunger and press down until the air pocket releases. Turn the syringe up so that the air moves towards the tip, and press until the plunger is touching the jello and the excess air is gone. Some of the jello is going to come out, deal with it. Collateral damage. Replace the caps and refrigerate for another hour. Keep cold until ready to serve!
I used Bacardi silver and Bacardi Mango Fusion for the rum.If you're wondering about switching out the pineapple juice, the only other thing I'd recommend is lemonade (and vodka), but really you should use pineapple juice. The opaque yellow color is perfect for getting just the right green for your shots.