Prepare each flavor of Jello separately according to package directions, but use half the amount of water called for. Pour each Jello into a separate 8x8 inch pan. Chill until firm, about 2-3 hours. Once firm, cut each Jello into 0.5 inch cubes.
In a small bowl, sprinkle the unflavored gelatin over the cold water and let it sit for 5 minutes to bloom.
Pour the 2 cups of boiling water into a large bowl. Add the bloomed gelatin and stir until completely dissolved.
Stir in the sweetened condensed milk until well combined. Let the mixture cool to room temperature.
Gently fold the Jello cubes into the cooled creamy Jello mixture.
Pour the mixture into a 9x13 inch serving dish. Chill for at least 4 hours, or until firm.
Notes
This recipe is best served chilled. You can use any combination of Jello flavors you like.