1.Heat oven to 350°F. Generously spray four 8- or 9-inch round cake pans with baking spray with flour.2.Make cake batter as directed on boxes. Divide batter equally among 4 bowls. Add food color to make 4 different shades of the same color. Pour batter into pans. Bake and cool as directed on box for 8- or 9-inch rounds.
3.In 2-quart saucepan, heat sugar and water to boiling. Cook to 245°F on candy thermometer.
4.Meanwhile, in large bowl, beat eggs whites, cream of tartar and salt with electric mixer on high speed until peaks form.
5.Reduce speed to low; slowly pour sugar syrup into egg whites. Increase speed to high; beat about 7 minutes or until sides of bowl are cool to the touch. Beat in vanilla.
6.Spread 1 tablespoon frosting on cake plate to act as glue. Place 1 cake layer on plate. Spread with thin layer of frosting. Repeat to use all layers. Frost cake completely with remaining frosting.