Combine egg whites, sugar, and salt in a heatproof mixer bowl set over a pot 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 a mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy and completely cool (test by touching the bottom of the bowl), about 10 minutes.
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. Keep buttercream at room temperature if using the same day. (If stored in an airtight plastic container, the buttercream will keep one week in the refrigerator; or 1 month stored in the freezer.)
Equally divide buttercream and place into three bowls. Flavor one bowl with 1/8 to 1/4 tsp (or to taste) Cherry Blossom extract and tint a very pale pink using the "rose" (pink) food coloring. Flavor the second bowl of buttercream with 1/4 to 1/2 teaspoon (or to taste) of the Violet Candy extract plus the Creme de Cassis and tint a very pale purple/lavender using the "violet" food coloring. Flavor the third bowl with 1/8 teaspoon (or to taste) of the Rose Petal extract and tint a very pale peach-pink using the "rose" and "ivory" food colorings. Cover all three bowls with plastic wrap and keep at room temperature if you are using the icing the same day. Refrigerate if using the next day.
Always start by tinting a smaller amount of buttercream than needed. Once the color is achieved, tint the remaining buttercream needed to complete the project.