Spray silicone skull molds generously with nonstick cooking spray and set aside.
In a small pot, add limeade and pour gelatin over, allowing it to rest for 1 minute. Over low heat, stir limeade and gelatin together until fully dissolved. Remove from heat and add in tequila, triple sec and lime juice. If using, add a drop or two of gel food coloring and stir until desired color is reached.
Pour mixture into closed skull molds, filling each mold to the top. Refrigerate for at least 5 hours until set, preferably overnight. When ready to serve, carefully unmold each skull and arrange on a plate over a layer of salt.
Equipment tip: To make all 12 jello shots at once, plan to have 3 skull ice molds on hand.