Prepare each flavored gelatin individually. Stir the green jello powder into 1 1/2 cups of hot water and chill overnight in a rectangular baking pan.

Repeat the previous step with the red jello powder and 1 1/2 cups of hot water.

Repeat the previous step with the orange or yellow jello powder and 1 1/2 cups of hot water.

In a separate bowl, sprinkle unflavored gelatin and sugar into 1/2 cup of cold milk. Stir and let stand for 5 minutes to bloom the gelatin.

Heat the remaining milk using a double boiler. Stir in the bloomed gelatin and sugar mixture and continue stirring until the gelatin grains are completely dissolved.

Stir in the remaining cold milk to cool the mixture.

Cut each of the chilled flavored jellos into 1/2 inch cubes.

Pour the milk jello into a mold (approximately 7 cups in size).

Carefully arrange the jello cubes in the milk jello, alternating colors to distribute them evenly. Avoid stirring to prevent the colors from bleeding.

Chill the cake for at least 2 hours, or until the milk jello is firm.

Cut the stained glass cake into slices and serve.