Spray a baking sheet with cooking spray and line with parchment paper. Spray parchment and sides of pan.
Place 1/2 cup cold water in a small bowl and sprinkle the gelatin over. Let sit.
until gelatin firms, about 3-5 minutes.
In a medium saucepan fitted with a candy thermometer, combine granulated sugar, 1/2 cup corn syrup and remaining 1/2 cup of water. Stir over medium heat until the sugar dissolves, then turn the heat up to high and stop stirring. Cook until the syrup reaches 240 degrees, about 6-8 minutes.
Add remaining 1/2 cup corn syrup to the bowl of a stand mixer fitted with a whisk attachment. With the mixer on low, add the gelatin, then slowly pour in the hot syrup down the side of the bowl. Gradually increase speed to medium high and whisk until mixture is light and fluffy and has tripled in volume, about 7-8 minutes.
Remove bowl from stand and drip about 5-6 drops each of pink and yellow food coloring and 4 of green food coloring gel. Use a rubber spatula to fold colors into streaks but do not not mix to completely combine. It should only take 1-2 folds. Spread onto prepared baking sheet and let set for 4-6 hours.
In a medium bowl, whisk to combine confectioners sugar and cornstarch. Use a sifter to coat the top of the fully set marshmallows with the mixture. Flip the pan onto a clean work surface, and dust with more of the confectioners sugar and cornstarch coating.
Using a 4-inch castle-shaped cookie cutter, cut the marshmallows into shapes.