Sprinkle gelatin over 120 grams of water a little bit at a time, whisking to combine. Try not to drop a lot of gelatin in one place to avoid large clumps.
Let gelatin sit for 15 minutes to bloom (absorb the liquid).
In a medium-sized bowl, combine white chocolate, condensed milk, and bloomed gelatin.
In a small sauce pan, boil 150 grams of water with the sugar and corn syrup until it reaches 217 F.
Pour boiled mixture over the white chocolate mixture.
Let sit for a few minutes until chocolate starts to melt.
Use an immersion blender to blend until smooth, keeping the blender entirely submersed to avoid bubbles.
Pour glaze through a fine sieve and separate into as many vessels as you would like colors.
Add food coloring to each vessel to create your desired colors.
Cover tightly with plastic wrap, pressing plastic wrap directly against the surface of each glaze, and cool to 95 F.
Place two drinking glasses on top of a cookie sheet and set your frozen cakes on top of the glasses.
Slowly but generously, pour one color glaze over the cake, being careful to avoid getting air bubbles.
Quickly, before the glaze fully hardens, pour any other colored glazes over the first one and gently mix colors however you'd like using an offset spatula.
Carefully pick up each cake up by using both palms and reaching under the cakes. To remove any glaze hanging down off the sides of the cakes, run a knife across the dangling glaze so that it sticks to the knife, and gently twirl the knife to remove. Place the cakes on serving plates.
To use the extra mirror glaze, heat it back up to 95 F in the microwave or set over a bowl of simmering water. If not using immediately, store in an air-tight container in the refrigerator.