Sift the icing sugar and almond flour together into a bowl, discarding any large pieces of almond flour left in the sieve. Repeat this process two more times until the mixture smooth and well combined.
Place the room-temperature egg whites in a large, grease-free steel mixing bowl. Beat with a handheld electric mixer until foamy, then add in the caster sugar a tablespoon at a time, beating between each addition.
Keep whisking until it forms stiff peaks. The meringue on the beaters should hold its shape in a stiff peak, with a slightly curled tip standing up tall and proud. You also should be able to turn the bowl upside down without the mixture falling out of it. Be careful not to under or overmix!
Add some gel food coloring. Use a little more than you would like, as the color will lighten.
Gently pour in the sifted almond flour and icing sugar mixture and carefully fold it in with the spatula. The mixture will deflate some, which is what we want, but be gentle. Keep folding until the mixture is smooth and perfectly well mixed. Most importantly, the mixture should fall in a smooth ribbon without breaking when the spatula is lifted from the mixture. You should also be able to make a complete figure 8 with the batter; if not, keep folding a little longer and test again. This process is called macaronage. Stop as soon as the batter is ready; under or over mixing here is potentially fatal! It's better to be a little undermixed than overmixed.
Gently spoon the mixture into a large piping bag fitted with a 1 centimeter round tip, twisting the top of the bag to seal.