Cut 2 empty milk cartons in half lengthwise to create molds for the stained glass jello. Clean the inside of the milk cartons thoroughly, dry them, and arrange them side by side on a baking tray.
Heat the lemon gelatin powder, 2 tablespoons of sugar, 1 tablespoon of unflavored gelatin powder, and 1 2/3 cups of water in a saucepan until the lemon gelatin powder has dissolved. Pour the hot lemon jello mixture into one of the milk carton molds. Repeat this process for the berry blue, raspberry, and lime jello flavors. Once all of the milk carton molds are filled with different flavors, transfer the baking tray to the refrigerator and let the jello set for 4 hours.
Stir together the water and unflavored gelatin powder and set aside briefly. Pour the heavy cream, sweetened condensed milk, and pineapple juice into a medium-sized saucepan and gradually heat the mixture. Add the gelatin mixture to the saucepan and continue to stir until the gelatin powder is completely dissolved. Once the pineapple cream is smooth, remove the saucepan from the heat and let it cool to 85°F.
After 4 hours, remove the stained glass jello from the fridge and cut each flavor into various shapes and sizes.
Place the silicone mold on a baking tray, spray the inside with a thin layer of non-stick cooking spray, and arrange half of the colorful stained glass jello pieces in the mold at random. Pour some of the cooled pineapple cream into the mold until the first layer of colorful jello pieces is covered. Transfer the baking tray to the refrigerator and let the first layer set.
Once the first layer of pineapple cream has become firm, arrange the remaining colorful jello pieces on top. Pour the rest of the pineapple cream mixture over the stained glass jello pieces, filling up the silicone mold. Transfer the baking tray to the refrigerator and chill for at least 4 hours.