Preheat the oven to 200ºC. Line a baking tray with baking paper and pour the caster sugar into the tray in an even layer. Put the tray in the oven for 5 minutes.
Meanwhile, whisk the egg whites on a low speed until you see bubbles, then increase the speed to medium until the egg whites form stiff peaks. You can check the egg whites are ready by turning the bowl upside down – if they don't slide out, that's stiff peaks.
By now the sugar will be ready. Take the tray out of the oven and reduce the oven temperature to 100ºC. Add the hot sugar to the egg whites mixture a tablespoonful at a time beating on full speed. Take care not to add any burnt or crystallised sugar from the edges of the tray. When all the sugar has been added continue to whisk for a further 5-7 minutes.Test the meringue by rubbing it in-between your fingers, keep whisking until you can't feel any grainy sugar. When the mixture is ready it will be bright white, thick and glossy.
Divide the meringue into bowls depending on how many colours you'll be using. Colour each bowl of meringue by adding a small drop of food gel and mixing well until you reach the desired colour. Fit a piping bag with a french tip nozzle, start at the top of the tree and pipe a line underneath, swirling as you go so that the lines of meringue get progressively wider and you have a tree shape.
Cut a couple of paper straws into 4 sections and push a straw into the bottom of each tree. Add sprinkles and bake for 2 hours or until the meringues are completely dry to the touch and lift away from the baking paper easily. Store in an airtight container for up to 2 weeks. Do not refrigerate. NOTES: Make sure your mixing bowl and utensils are spotlessly clean, any trace of grease in the bowl will deflate your meringue. When you separate the egg whites from the yolks make sure there is no trace of yolk or eggshell in the whites. Crack the eggs into your clean hand and let the whites slip through your fingers – you will catch the yolk in your palm and it will stay intact. Don’t add the sugar until the egg whites are at stiff peaks or you’ll deflate the mixture. You want to get as much air into the mixture as possible. In the final stages of making meringue keep mixing until the meringue is completely smooth when felt between your fingers. The bowl will feel much cooler to the touch on the outside and the meringue will look bright white, thick and glossy.