Divide 1 ½ cups of the sugar among zip seal baggies, 1 bag for each colour you plan to use. Add a drop or two of food colouring to each bag, press most of the air out of the bag, and seal with the zipper. Gently massage the sugar around to distribute the food colouring evenly throughout the sugar, taking care to break down any clumps.
Once the colour is evenly distributed with no clumping, transfer the sugars to individual little bowls, for dipping
In a small, microwave safe bowl, sprinkle the gelatin over ¼ cup of water, set aside.
In medium saucepan, combine remaining cup of sugar, with remaining ¼ cup of water and salt. Cook over medium heat, stirring constantly, until sugar is dissolved. Bring to a boil and continue to cook – without stirring – until mixture reaches 240°F on a candy thermometer. Remove from heat and add gelatin mixture, stirring until dissolved and completely combined, with no lumps of gelatin.
Transfer sugar syrup to the bowl of a stand mixer, allow it to cool for about 5 minutes.
Beat sugar syrup in a standing mixer (or with a hand held electric mixer) on medium speed until it becomes opaque/white.
Once syrup is no longer clear, add the vanilla and turn the mixer speed up to high. Continue whipping the syrup until it is thick, white, glossy, and almost triple the original volume. This may take up to 10 minutes.
Carefully spoon the marshmallow into a large pastry bag. Cut a small amount off the end of the pastry bag, so the opening is maybe ½″ across.
Working quickly, insert the end of the pastry bag into one of the mini ice cream cones, gently squeezing while pulling the bag slowly out of the cone, allowing it to fill completely with marshmallow. Let the marshmallow form a bit of a mound on top of each cone, then stop squeezing and pull bag completely away from the cone (you may need to swirl it a bit to avoid a lot end on it!)
As you finish piping each cone, gently roll the top of the cone in the sugar color of your choice, taking care to fully coat all exposed marshmallow.
As you dip each, lay them to rest on a baking sheet lined with parchment paper. After a few minutes, the marshmallow may have slid a little – I like to re-dip them if needed, and rotate / reposition each cone to even out the “slide”.
Let the cones set up for a 2-3 hours, then transfer to an airtight container or zip bags to prevent them from drying out. These will last about 3 days, kept at room temperature.