Prepare the oven.
Preheat the oven to 200°F. Line baking sheets with parchment paper.
Whip the egg whites.
Place the egg whites in a clean, dry mixing bowl. Using a stand mixer fitted with the whisk attachment or a hand mixer, beat on medium speed until foamy, then continue until soft peaks form.
Gradually add the sugar.
With the mixer running, add the sugar slowly, about 1 tablespoon at a time, allowing each addition to mix in before adding more.
Add the vanilla and vinegar. Increase to medium-high speed and continue whipping until the meringue is thick and glossy with stiff peaks.
Rub a small amount between your fingers. It should feel smooth rather than gritty. If sugar grains remain, continue mixing and check again.
Add color if desired.
Gently mix in a small amount of gel food coloring until evenly blended. For a striped effect, brush thin lines of gel coloring inside the piping bag before adding the meringue.
Pipe the cookies.
Transfer the meringue to a piping bag fitted with your preferred tip, or a sturdy zip-top bag with one corner snipped off.
Pipe 1-inch kisses onto the lined baking sheets, leaving a little space between them.
Bake low and slow.
Bake for about 2 hours, until the cookies are dry and release easily from the parchment. Larger cookies may need additional drying time.
Turn off the oven, crack the door slightly, and leave the cookies inside for 1 hour.
Cool and store.
Allow the cookies to cool completely, then promptly transfer to an airtight container.