Spoon three-quarters of the meringue into a piping bag fitted with a 1cm round piping nozzle. Set aside. Add the red food colouring to the remaining meringue in the mixing bowl and whisk until the colour is even. Spoon into another piping bag fitted with a 1cm round nozzle.
Hold the piping bag of white meringue upright and pipe 20 mushroom stems by squeezing the piping bag and lifting upwards at the same time to make the meringue stand tall.
For the mushroom tops, pipe 10 red round domed discs roughly 3cm in diameter and 10 white discs the same size.
Bake for 90 minutes. The meringues should be hard, dry to the touch and easily lift off the baking paper.
Close the oven door and turn the oven off completely. Let the meringues cool down inside the oven for at least 2 hours or overnight (this will prevent them from cracking when you assemble them).