Preheat the oven to 170/C/325F/Gas 3 and line a large baking tray with baking paper.
Put the icing sugar, ground almonds and 40g egg whites together in a large bowl and mix to a paste.
Put the water and caster sugar in a small pan and heat gently to melt the sugar, then turn up the heat and boil until the mixture starts to go syrupy and thickens.
Whisk the remaining 50g egg whites in a small bowl until medium-stiff peaks form when the whisk is removed from the bowl, then pour in the sugar syrup, whisking until the mixture becomes stiff and shiny. Add a few drops of food colouring of your choice (*to make a selection of different colours, you will need to duplicate this receipe). Tip this meringue mixture into the almond paste mixture and stir gently until it becomes stiff and shiny again.
Spoon into the piping bag. Pipe a little mixture under each corner of the baking paper to stop it sliding around. With the bag held vertically, pipe 4cm/1½in flat circles onto the lined tray, about 2cm/¾in apart, twisting the bag after each one. The mixture should be quite loose to give a smooth finish. The piping will leave a small ‘tip’ on each circle so, when they’re all piped, give the tray 2–3 slams on a flat surface to flatten them.
Leave to stand for 30 minutes to form a skin then bake in the oven for 12–15 minutes with the door slightly ajar until firm. Remove from the oven, lift the paper off the baking tray and leave the macaroons to cool on the paper.
When cool, sandwich the macaroons together with whipped cream. They can be kept for a couple of days, if they hang around that long!