Preheat oven to 200º F. Line two baking sheets with parchment paper.
Add egg whites and sugar to the bowl of a standing electric mixer. Whisk to combine. Set bowl over a saucepan filled with just enough simmering water to heat the egg mixture without touching the bottom of the bowl. Stir constantly with a silicone spatula, scraping down the sides frequently until the sugar melts and reaches 140º F (once the mixture begins to lose its grittiness, check frequently with an instant-read thermometer). Carefully transfer the hot bowl to a mixer fitted with the whisk attachment. Whisk on high speed until the meringue is fluffy and has cooled to room temperature (this will take some time).
In the meantime, whirl strawberries in the bowl of a food processor until well pulverized. Let the dust settle a bit before opening the lid. Sift the strawberries through a fine mesh sieve and discard seeds and other large bits.
Once the meringue has cooled, add the sifted strawberry dust. Mix at slow speed to combine for about 30 seconds before adding vanilla extract and food coloring, if using. Whisk on high speed until evenly combined.
Fit a pastry bag with a 1/2-inch round tip and fill the bag with the meringue. Remove parchment paper from the baking sheets and add a dab of the remaining meringue on the spatula or whisk to the four corners of each baking sheet (this will help the parchment stick to the baking sheets during piping). Replace parchment paper and smooth it down over the sticky meringue.
To pipe hearts, create a "V" shape using the following technique: pipe a round dollop about 1-inch wide and pull away from it at an angle, releasing the pressure slowly as you drag the tip away. Pipe a second round dollop next to the first, releasing and pulling toward the tip of the other half, overlapping slightly as you go. (If you're new to piping --like me--it may take some practice to align the piping pressure and movement to achieve the right shape.) If you have remaining meringue after filling your baking sheets, fill spaces between hearts with small dollops, if desired.
Bake meringues in preheated oven until crisp but not browned, about 2 to 3 hours. Let meringues cool completely in the oven with the heat turned off. Store in an airtight container for up to a week.