Lightly spray a 9 x 13-inch baking pan with nonstick cooking spray.
In a 4-cup glass measuring cup, add 3 packets of the Knox unflavored gelatin. Add a ¼ cup of cool water, stirring to moisten the gelatin, then allow the gelatin to soak for 5 minutes. After 5 minutes, add 1¼ cup boiling water, stirring with a whisk until completely dissolved. Next, stir in the two cans of sweetened condensed milk. Add more hot water, if needed, to make a total of 4 cups. Set aside to cool.
In a 2-cup glass measuring cup, add 1 packet of Knox unflavored gelatin. Add 2 tablespoons of cool water, stirring to moisten the gelatin, then allow the gelatin to soak for 5 minutes. After 5 minutes, add 1½ cups of boiling, stirring until the gelatin is completely dissolved. Then pour in the box of blue jello, stirring until dissolved. Let cool for 20 minutes, then pour into the prepared 9 x 13-inch pan. Place the pan in the refrigerator, make sure it is sitting level and flat, and allow the jello to set up for 30 minutes.
When the first jello layer is set firm, leave the pan in the refrigerator if possible, and measure 1 cup of the sweetened condensed milk mixture. Pour slowly and carefully over the first jello layer, making sure the entire blue layer is covered. Allow to set up for 20 to 30 minutes or until firm.
While the sweetened condensed layer is setting up, prepare the green layer of jello, repeating the instructions in step 3. **Before adding the green layer, make sure the sweetened condensed milk layer is set firm. For the remaining layers, alternate the milk mixture with a jello layer (alternating steps 3 and 4). For the remaining jello layers, the order is yellow, orange, and red. Remember to allow the jello to cool for 20 minutes before adding it to the pan. Allow each layer to set for at least 30 minutes before adding the next layer.**If possible, leave the pan in the refrigerator while pouring the layers. As the pan gets fuller, it gets challenging to carry it back and forth to the refrigerator without spilling.
Once all of the layers have been added, allow the finished jello to set up for at least 2 hours or, even better, overnight before attempting to cut and remove.
Once completely set, cut into 1-inch to 1½-inch squares.