Preheat oven to 175 degrees. Place egg whites, sugar, and cream of tartar in a heatproof mixer bowl set over a pan of simmering water. Whisk until sugar dissolves, about 2 minutes. Transfer to a mixer, and beat with mixer on medium-high speed until stiff, glossy peaks form, about 7 minutes. Divide meringue into batches, and tint each with food coloring as desired. Transfer to pastry bags fitted with medium open star tips. Pipe hearts (1/2 to 1 inch each) onto parchment-lined baking sheets. Bake for 2 hours. Turn oven off, and let meringues stand in oven for 8 hours (or overnight).