First, you'll bring 5 cups water to a boil. Combine 1 cup boiling water with 1 package of Jello, stirring until dissolved. Place the dissolved Jello in one of the prepared containers (spray containers down with non-stick cooking spray before pouring in Jello mixture). Do this with the remaining packages of Jello. Refrigerate for at least 4 hours, or overnight.
When Jello has set, bring another 1 1/2 cup water to a boil. While water is heating, sprinkle 2 packets of unflavored gelatin over 1/2 cup of cold water. Allow to stand for approximately 4 minutes. When water is boiling, pour 1 1/2 cups boiling water over the dissolved gelatin and stir to combine. Add the sweetened condensed milk and mix well. Allow to come to room temperature.
Spray a 9 X 13" pan with non-stick cooking spray. Carefully cut the colored Jello into cubes and gently toss to combine in the pan.
Pour the cooled milk mixture over the colored Jello and rearrange some of the colored Jello, if necessary.
Place the pan in the refrigerator and chill overnight. Cut into squares or various shapes using cookie cutters!