Mirror, mirror on the wall, who is the shiniest cake of all? Thanks to gelatin, it’s this one!
Line rimmed baking sheet with parchment paper and place cooling rack inside baking sheet. Place frosted cake on 9-inch cardboard round and place in freezer while making glaze.
In small bowl, use spoon to stir together gelatin and ¼ cup water. Let sit for 5 minutes.
Meanwhile, in small saucepan, combine sugar, sweetened condensed milk, vanilla, and remaining ½ cup water. Cook sugar mixture over medium heat, stirring occasionally with rubber spatula, until mixture starts to bubble around edges, 5 to 8 minutes. Turn off heat and slide saucepan to cool burner.
Use rubber spatula to scrape gelatin mixture into sugar mixture. Stir until gelatin is dissolved. Add white chocolate chips and stir until melted and mixture is smooth, about 1 minute.
Place fine-mesh strainer in medium bowl. Pour chocolate mixture through strainer (ask an adult for help—saucepan will be heavy); discard solids. Let glaze cool for 15 minutes, stirring occasionally to prevent skin from forming.
Pour ½ cup glaze into each liquid measuring cup. Add gel food coloring to each glaze and combine in bowl following photos, below.
Add black food coloring to one liquid measuring cup and stir to combine. Clean spatula. Add purple food coloring to second liquid measuring cup and stir to combine. Clean spatula. Add turquoise food coloring to remaining glaze in bowl and stir to combine.
Pour black glaze into center of turquoise glaze in bowl (do not stir together). Pour purple glaze right next to black glaze in bowl (do not stir together).
Remove cake from freezer and place on cooling rack set inside parchment-lined baking sheet. Working quickly, pour glaze over top of chilled cake (in center of cake), letting glaze drip down and coat sides of cake. Some glaze will drip onto baking sheet—that’s OK!