Pre-chill 9X13 baking dish in refrigerator for at least 30 minutes. Do not grease pan.
In 4-cup glass measuring cup, pour 1 1/2 cups boiling water. Add three packets of Knox gelatin, whisking constantly until gelatin is dissolved. Add in both cans of sweetened condensed milk, stirring to combine. Add more water if needed to reach 4 cups of the milk mixture.
In 2-cup glass measuring cup, pour 1 1/2 cups boiling water. Add in one packet of Knox gelatin, whisking constantly until gelatin is completely dissolved. Pour in 1 large package of Berry Blue Jello gelatin, whisking until completely dissolved. Let cool then pour into chilled baking dish before returning to refrigerator for 20 minutes or until layer is set.
Using a clean and dry measuring cup, measure out 1 cup of the milk mixture then pour over the Jello layer. Return dish to refrigerator for 20 minutes or until layer is set.
Repeat steps 3 and 4 for each additional Jello flavor.