Beat the egg whites, salt and cream of tartar, slowly add in 1/3 cup of sugar and continue beating until soft peaks form.
As the eggs are beating add the remaining sugar and 1/3 cup water to a medium saucepan then place on medium-high heat.
Stir until sugar melts and becomes clear.
Maintain at medium-high heat until temperature reads 235-240F.
Drizzle the sugar into the mixer immediately.
Run mixer until meringue is cool/tepid.
Add room temperature butter into running mixer one tablespoon piece at a time.
Add the salt and vanilla if using
Beat until butter is combined and mixture has reached a silky consistency.
Divide into 5 bowls. Keep one bowl white. Add food coloring to each batch to achieve a gradient look.
Whip the frosting and transfer to separate piping bags JUST before using.