Preheat your oven to 120°C. Line a baking tray with baking paper.
Place the egg whites in a large bowl. Whisk until stiff peaks form when the beater is lifted. Add the castor sugar one tablespoonful at a time, whisking well between each addition. The mixture should have a smooth texture, not grainy. Whisk in the salt and vanilla extract.
Place a drop or two (see cook's notes above) of gel food colouring inside the piping bag near the nozzle. Use a skewer to scrape the drop up the side of the piping bag, i.e. the bag should have streaks of colour running up the sides. Spoon the meringue into the bag.
Pipe the meringues onto the tray, allowing some space between for spreading. Bake for 1 hour. Switch the oven off, keep the oven closed and leave the meringues for a further hour.
Allow the meringues to cool. Store in an airtight container.