Dissolve each box of jello into its own small plastic container (using 1 cup of boiling water per flavor) and chill overnight or until Jello has set (at least 4 hours).
Cut each of the different flavors into small squares or cubes. Using a 9x13 glass dish, mix the cubes gently together. You just want to mix them up, so be very careful not to break them up.
In a medium-sized bowl, dissolve 2 envelopes of unflavored gelatin in 2 cups of hot water. Let the hot mixture cool slightly and then mix in the sweetened condensed milk.
Let cool completely (it should be cool to your touch). Pour the condensed milk and gelatin mixture over the cubes of Jello.
Chill in the refrigerator overnight.
Cut and serve.
Tip: Do not use the traditional 2 cups of water that you typically use when making jello for the jello cubes! Be sure to let the condensed milk mixture cool COMPLETELY or it will melt the colored Jello cubes.Got a bundt pan? You can make it in there for a really pretty Broken Glass Jello mold.