Lightly coat a 13 x 9-inch pan with cooking spray. Dissolve one package of flavored gelatin in 1 cup boiling water, cool it briefly to room temperature, pour it into the pan, and refrigerate for 25 to 35 minutes, or until firm.

For the creamy layer, sprinkle the two envelopes of unflavored gelatin over 1/2 cup cold water and let it stand for about 1 minute. Add 1 1/2 cups boiling water and stir until the gelatin is completely dissolved, then stir in the sweetened condensed milk and cool the mixture to room temperature.

Pour enough of the cooled milk mixture over the first set colored layer to create an even white stripe about 3/4 to 1 inch deep. Refrigerate until firm, usually 25 to 35 minutes, making sure the mixture is not warm when it touches the colored gelatin.

Dissolve the next flavored gelatin package in 1 cup boiling water and cool it to room temperature. Pour it slowly over the white layer, refrigerate until firm, and continue alternating colored gelatin and white gelatin until all mixtures are used.

Refrigerate the completed dessert for at least 4 hours, preferably overnight. Dip a thin knife in warm water, wipe it dry, and cut the gelatin into approximately 24 squares, warming and wiping the knife between cuts for tidy edges.