Prepare the flavored gelatin according to package instructions.
When the gelatin is firmly set, cut it into 1-inch cubes. Place the cubes in a lightly oiled 9- by 13-inch glass baking dish, evenly distributing the colors, then refrigerate.
In a medium bowl, sprinkle the unflavored gelatin into 1/2 cup cold water. Let it sit for 2 minutes. Add 1 1/2 cups boiling water, stirring until the gelatin is dissolved. Stir in the condensed milk. Refrigerate the mixture for 30 minutes.
Slowly pour the cooled milk mixture over the cubes. To remove bubbles on the surface, blow on them with a straw. Refrigerate the dish overnight.
Slice the finished concoction into 2-inch squares; for best results, dip a very sharp knife into hot water, then wipe it dry before making each cut. With a spatula, lift each slice onto a cutting board, then slice it in half horizontally before placing it on the serving platter cut side up.