Spray a 9x13 pan generously with cooking spray. You could use a smaller 9x9 pan if you needed to, but then you would have to cut out 3 of the layers. I recommend cutting out the bottom 3 - grape, milk, and blue raspberry. In that case, you would start and end with red.
In a small bowl, whisk together the box of grape Jello with 1 teaspoon of the unflavored gelatin and 1 cup of boiling water until you are certain that all the powder is dissolved. Pour into the prepared pan, set in the fridge ensuring it is completely flat, and chill in the refrigerator to set (about 30 minutes).
As soon as the grape Jello goes into the fridge, do this step. In a medium bowl, whisk the sweetened condensed milk with 2 cups of boiling water. In a smaller bowl, sprinkle 1 ounce (4 envelopes) of unflavored gelatin over 1 cups cold water and whisk to dissolve. Allow the mixture to rest for 5 minutes so that the gelatin can absorb as much water as possible. After 5 minutes, pour 1 cup boiling water into the unflavored gelatin mixture and stir, then pour the gelatin mixture into the sweetened condensed milk mixture and stir again.
Mix up the rest of the jello. Pour both boxes of red jello into a bowl with 1 full packet of unflavored gelatin and 2 cups of boiling water. Stir well until all the powder is dissolved. Repeat with the blue jello. Allow to cool on the counter but do not add ice or any more water.
Once the grape layer is completely set, remove it from the fridge and pour 1 cup of the sweetened condensed milk mixture over the top. (If you want precise, even layers, make sure the milk mixture is room temperature before adding to the grape layer. If you like wavy, uneven layers, it's okay to pour it on hot.) Return to the refrigerator for at least 15-20 minutes to set.
Repeat the layer process, with 1 cup of blue jello first, alternating with 1 cup of white gelatin between each colored layer. (It will be purple, white, blue, white, red, white, blue, white, red.) Remember, if you want precise, even layers, you must make sure that all of the Jello mixtures cool to room temperature before pouring over the previously set layer.
Once all the layers have been set, cover with plastic wrap and refrigerate at least 4 hours to set completely.