Add the icing sugar and almonds to a food processor and whizz for 2 minutes.
Sieve the mixture into a large bowl.
Put the almond mixture back into the food processor and whizz for a further minute.
Re-sieve the mixture into the large bowl and leave to one side.
Whisk the egg whites until they start to form soft peaks. I did this using the whisk attachment on my KitchenAid but you could use a hand mixer too.
Gradually add in the caster sugar 1 tablespoon at a time. Leaving a minute or so in-between each addition.
Once all the caster sugar has been added continue to whisk the meringue until they form stiff peaks and are glossy and shiny.
Gently fold the almond mixture into your egg whites until they are just combined.
Split the mixture equally into two bowls. To one bowl add the red food colouring and to the other bowl add the blue food colouring.
Press the mixture into the sides of the bowl. This is called macaronage and you are trying to get the mixture to be as smooth as possible as well as incorporate the food colouring. You know it is ready when it flows like molten lava from your spatula.
Add each coloured mixture to a separate piping bag with a small round nozzle and leave them to one side.
Line your baking trays with a macaron mat or baking parchment
Pipe the batter onto the mats or baking parchment. You need to hold the piping bag completely vertically or you will end up with wonky feet. Pipe the macarons around an inch or so apart so that they don't touch.
Tap the trays on your worktop to knock out any air bubbles. If you see any air bubbles on top gently pop these with a skewer.
Sprinkle over the sprinkles and leave the macarons for 45 to 1 hour so that a skin is formed and they are completely dry to the touch.