Lightly grease two cake pans.
Place the gelatin in the bottom of the bowl of a stand mixer. Add ½ cup cold water and set aside.
In a medium-sized pan set over a high heat, heat the sugar, corn syrup, salt, and vanilla, stirring occasionally. When the mixture comes to a full rolling boil, stop stirring, and instead swirl the pan occasionally by the handle. Attach a candy thermometer to the pot and let the mixture heat to 240 degrees F. This will take just over 5 minutes.
Using the whisk attachment of your stand mixer, turn the mixer on to medium speed and gradually pour the hot syrup into the bowl. When all the syrup has been added, pour in another ½ cup of water.
Increase the speed to high and beat for 15 minutes, until the marshmallows are thick and fairly stiff.
Divide your marshmallow mixture into as many parts as the number of colors you want to use. Beat in the food coloring, adding as little or as much as you want, depending on whether you want the marshmallows to be pastel-colored or really bright.
Grease your fingers and spoons, and transfer the marshmallow mixture into the prepared pans.
Let the marshmallows set for at least 4 hours, or overnight.