Mix purple gelatin with 1 cup (0.24 l) boiling water. Stir well and pour into a 9×13 pan. Refrigerate for 30 minutes. Repeat with the remaining colors, pouring on top of each layer, ending with red. Refrigerate for 1 hour.
3 oz cherry gelatin, 3 oz orange gelatin, 3 oz island pineapple gelatin, 3 oz lime gelatin, 3 oz berry blue gelatin, 3 oz grape gelatin, water
Add gelatin and 1/2 cup of cold water in a bowl. Let set for 5 minutes. Add 1-1/2 cups boiling water and mix well until dissolved completely. Stir in sweetened condensed milk and mix well. Set aside for about 30 minutes to cool.
.5 oz unflavored gelatin, 14 oz sweetened condensed milk
Cut gelatin into 1/2" cubes and pour into a separate 9×13 pan. Pour white gelatin over rainbow jelly and refrigerate for 2 hours or until firm. Cut into 32 cubes with a knife and serve.
Video
Notes
Feel free to leave out the sweetened condensed milk. The rainbow cubes are adorable as is, but the white background enhances the "stained-glass" look.