Combine egg whites, sugar, and salt in the heatproof bowl of a standing mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).
Attach the bowl to the mixer fitted with the whisk attachment. Start on low speed and gradually increase to medium-high speed, whisking until stiff peaks form and the mixture is fluffy and glossy, about 10 minutes.
Set aside ½ cup of the meringue. You’ll use this later to add color to the frosting.
With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Once all butter has been added, whisk in vanilla. Switch to the paddle attachment, and continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down sides of bowl with a flexible spatula, and continue beating until the frosting is completely smooth.
Set aside 2 cups of white buttercream.
To tint remaining buttercream, separate the reserved ½ cup of meringue into small bowls, one for each unique color. If you’re making ombre in one color, you only need one batch of tinted meringue. Add gel coloring to the meringue and stir until you reach a deep shade of your desired color (color will become lighter when you mix it into the buttercream).
Separate buttercream into your desired number of colors. If you’re making ombre, separate into 3 batches so you can tint each batch at a different color intensity (from lighter to darker).
Add tinted meringue to buttercream frosting and stir with a flexible spatula, adding additional tinted meringue until desired color is achieved.
Transfer frosting to pastry bags fitted with a coupler (no tip required as you’ll be piping thick rows of frosting). Frost cake, serve, & enjoy!