Oven temperature must be accurate and remain low to keep meringue from cracking or browning. To make enough roses for the Ribbon Rose Cake, you may need to make this recipe three times.
Have a pan of simmering water ready. Combine egg whites, sugar, and meringue powder in heat-proof bowl of electric mixer. Set bowl over simmering water. Whisk until sugar dissolves and whites are hot, 3 to 4 minutes.
Transfer bowl to electric mixer. Using whisk attachment, mix on low speed, increasing to high, until stiff, glossy peaks form, about 10 minutes; add vanilla.
Preheat oven to 175 degrees. Make meringue roses: Tint a single recipe of meringue with food colors as desired. Use Ateco petal tips #102, #104, and #125 for various rose sizes. Fill 2 pastry bags, each fitted with a coupler, with tinted meringue. Using a dot of meringue, attach a 1 1/2- inch square of waxed paper to a number 7 flower nail. Holding nail between thumb and forefinger, twirl it clockwise as you hold the bag at a 45-degree angle and press out icing. Slide waxed paper off nail; place on baking sheet. Continue, placing only same-size roses on each tray. Bake trays of roses for 30 to 60 minutes or until dried through (baking times will vary with size). For the Ribbon Rose Cake, make 5 dozen roses in various sizes: 1 dozen 1 1/2-inch-diameter roses, 2 dozen 1-inch roses, and 2 dozen 1/2-inch roses. Cool completely; store in airtight containers.