Lightly coat a large glass dish or mold with cooking spray or a thin layer of neutral oil, then set aside.

Prepare the first flavored gelatin by dissolving 1 box in 1 cup boiling water, stirring until fully dissolved, then add 1 cup cold water. Pour a thin layer into the prepared dish and refrigerate until just set, about 20 to 30 minutes.

To make a creamy layer, sprinkle 1 packet of unflavored gelatin over 1/2 cup cold water in a bowl and let it bloom for 5 minutes. Stir in 1 cup boiling water until dissolved, then whisk in 1/3 of the sweetened condensed milk, 1/3 of the sour cream, and a little vanilla until smooth.

Carefully spoon or pour a thin layer of the creamy mixture over the set gelatin layer. Refrigerate until set enough to support the next layer, about 20 to 30 minutes.

Repeat the process with the remaining flavored gelatins and creamy mixture, alternating layers and chilling each one until set before adding the next. Work gently so the layers stay distinct.

After the final layer is added, chill the dessert for at least 4 hours, or until completely firm.

To unmold, briefly dip the bottom or sides of the mold in warm water if needed, then invert onto a serving plate. Slice and serve chilled.