In a small bowl, whisk gelatin and 4 tbsp cold water and set aside until the gelatin absorbs the water and solidifies. In a separate bowl, whisk egg yolks and sugar.
In a small pan, heat the milk on medium-low and dissolve the gelatin mixture into the milk. Slowly add the milk to the egg and sugar mixture and set aside.
In a small bowl, dissolve matcha in the warm water and add to the gelatin mixture. Whisk thoroughly while cooling the bowl in ice water.
In a large bowl, whisk heavy cream until medium peaks form. Carefully fold the whipped heavy cream into the matcha mixture, taking care not to overmix.
Remove the sponge cake from the fridge and carefully remove it from the pan. The sponge cake will be the base for the mousse. Depending on how tall you want your cake to be, cut at least 1 in/2.5 cm through the bottom of the cake with a serrated knife.
With a springform pan or a cake pan, put the base sponge layer on the bottom, and fill the rest of the pan with mousse. Work quickly on this step before the mousse sets. Chill in fridge for several hours until the mousse is set.
Mirror Glaze
Whisk gelatin and 1/2 of the cold water in a bowl and set aside until the gelatin absorbs the water and solidifies.
In a small saucepan over medium heat, combine the remaining 1/2 cup water, sugar, and condensed milk. Stir until mixture dissolves and bring to a simmer. Remove from heat and add the solidified gelatin and whisk until dissolved.
Add white chocolate and let the remaining heat soften the chocolate for several minutes. Whisk slowly until smooth.
Divide the glaze into separate bowls, each containing a different color. One bowl should be a larger portion to contain a base color (royal blue or sky blue), while the smaller bowls will contain accent colors (purple, pink, white).
Whisk a few drops of food coloring into its designated bowl. Let the glaze cool down until 90°F/32°C, or until it barely feels warm when touched. (This is very important!) Stir frequently to prevent it from settling.