Pour the red Jell-O into a mixing bowl. Microwave 2 cups of water on HIGH for 2 minutes. Pour boiling water over the red Jell-O; stir until dissolved about 2 minutes.
Pour red Jell-O into an 8x8-inch dish sprayed with cooking spray. Refrigerate for 2-3 hours or until firm. Repeat steps with blue Jell-O.
Pour 1/2 cup cold water into a mixing bowl. Sprinkle unflavored gelatin over the cold water; let stand 1 minute. Microwave 1 1/2 cups of water on HIGH for 2 minutes. Stir in boiling water. Add condensed milk; stir well. Cool to room temperature, but not until set, about 25-30 minutes on the counter OR a quick chill in the refrigerator for 15-20 minutes.
Meanwhile, cut red and blue Jell-O into cubes. Randomly place the cubes in a 13x9-inch pan sprayed with cooking spray.
Pour cooled milk mixture over the cubes; gently stir to evenly distribute the Jell-O cubes. Refrigerate for 2-3 hours or until firm before cutting into cubes or shapes.