Whisk the egg whites, sugar and salt in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water)until the mixture is warm and the sugar dissolves.Remove the bowl from the pan; let cool slightly.

Beat egg white mixture in a stand mixer fitted with the whisk attachment (or a hand mixer) on medium high speed until stiff peaks form, 12-15 minutes. Beat in the butter a few pieces at a time, then continue beating until smooth. (Dont worry if the mixture looks sepperated at first, it will come together as you keep beating).
Color as desired.