Whisk cornstarch and 3/4 cup sugar in a saucepan; pour boiling water into cornstarch mixture, place over medium-low heat, and whisk until mixture thickens and becomes clear, about 5 minutes.
Beat egg whites with salt in a large metal or glass bowl, using an electric mixer, until egg whites hold stiff peaks, 3 to 5 minutes. Lift your beater or whisk straight up: the egg whites will form sharp peaks. Beat 3 tablespoons sugar into egg whites, followed by vanilla extract. Slowly pour hot cornstarch mixture into egg whites and beat constantly until mixture is stiff.
Pour whipped filling into pie crust and sprinkle with colored Chill until set.