Whisk sugar, egg whites, and salt in a heatproof mixer bowl set over a pan of simmering water until sugar dissolves and syrup registers 160 degrees on a candy thermometer.
Transfer bowl to mixer, and whisk sugar syrup on medium speed for 5 minutes. Raise speed to medium-high, and whisk until stiff, glossy peaks form, about 6 minutes.
Reduce speed to medium. Add butter, 1 piece at a time, whisking well after each addition. Whisk in food coloring.