1. Preheat oven to 225 degrees. Put egg whites, sugar, and cream of tartar into the heatproof bowl of an electric mixer set over a pan of simmering water.Whisk until mixture is warm and sugar has dissolved.

2. Attach bowl to mixer fitted with the whisk attachment. Starting on low speed and gradually increasing to high, beat until stiff, glossy peaks form, about 5 minutes. Beat in vanilla.

3. Transfer half the meringue to another bowl. Using a rubber spatula, stir orange zest into 1 bowl of meringue. Stir grapefruit zest and pink food coloring into remaining meringue.

4. Place a 3 1/4-by-3 1/2-inch heart-shape cutter in one corner of a parchment-lined baking sheet. Using a small spatula, fill heart with meringue; swirl. Push down edges of meringue with the spatula, and lift up cutter. Repeat, spacing meringue hearts 2 inches apart on baking sheets.

5. Bake until meringues are firm and can be removed easily from parchment paper, 2 1/2 to 3 hours. Meringues can be stored between layers of parchment paper in an airtight container at room temperature up to 3 days.