In a small bowl, add 1 1/4 cups of the boiling water to 1 package of the gelatin and stir until the gelatin is fully dissolved. Let the mixture cool to room temperature, then pour 3/4 cup of the dissolved gelatin mixture into a 7-cup mold. Refrigerate until set but not firm (about 15 minutes). Add 3 tablespoons of the sweetened condensed milk to the remaining gelatin and stir until the mixture is smooth. Let the mixture cool to room temperature, then pour over the clear gelatin layer in the mold. Refrigerate until set but not firm (about 15 minutes).
Repeat the process for each flavor to create 10 layers, 1 clear and 1 opaque layer of all 5 Jell-O colors. Remember that the gelatin must cool to room temperature or cooler before spooning it over the layer already in the mold. It may be best to use 2 small bowls for each flavor, one for the clear and one for the opaque gelatin to cool in before pouring each into the mold. Have 2 additional small bowls on hand so you can begin to prepare the next flavor layers while the preceding layer is cooling.
Refrigerate the finished mold until firm. Unmold and serve.
Reprinted with permission from Hello, Jell-O!: 50+ Inventive Recipes for Gelatin Treats and Jiggly Sweets. Copyright (c) 2012 by Victoria Belanger. Published by Ten Speed Press, an imprint of the Crown Publishing Group, Berkeley, CA.