Gel colours work much better in buttercream icing than liquid food colouring because they give more colour without making the icing sloppy. If you want an extra silky buttercream swap the milk for ...
To make the icing, lightly whisk the egg whites adding the sugar at intervals. Beat well until the icing forms soft peaks when the whisk is removed. Add the glycerine, if using, and the lemon juice.
For the royal icing: In the bowl of a stand mixer fitted with a paddle attachment, combine the powdered sugar, egg whites, and lemon juice. Beat on low speed until combined, then increase to ...