Preheat the oven to 200°F/93°C and line a large baking sheet with parchment paper or a silicone mat. Set aside.
Before you whip up the meringue, make sure there are no traces of grease on the whisk attachment or in your bowl. Grease can prevent the meringue from whipping up properly. If you see or feel any grease, moisten a paper towel with vinegar and wipe the inside of the bowl and the whisk attachment to remove any grease.
Then add 4 large, room-temperature egg whites into the bowl of a stand mixer and add 1/4 tsp cream of tartar. Mix on medium speed with a whisk attachment until the surface is covered in tiny bubbles. Note: Make sure the egg whites are fully at room temperature. If they're cold, they won't whip up half as well.
Gradually mix 1 cup of granulated sugar, 1 Tbsp at a time, into the egg whites over a few minutes while mixing on a medium-low speed. Adding the sugar gradually makes a huge difference in the stability of the meringue. Increase the mixing speed to a medium-high speed (6 on a KitchenAid). When the meringue gets close to reaching stiff peaks, add 1/4 tsp of rose water or vanilla extract and a drop of pink gel food coloring. If you want a slightly warmer pink, you can also add a tiny drop of yellow gel food coloring (about 1/4 of the amount of pink food coloring). Keep mixing until stiff, glossy peaks form.
Scoop almost all of the pink meringue into a large piping bag fitted with an open star piping tip (like a Wilton 1M), leaving about 1/4 cup of meringue in the bowl. Seal the top of the bag with a clip or rubber band.
Color the remaining meringue a warm green using a generous squirt of neon green gel food coloring. Scoop it into a piping bag fitted with a leaf piping tip (like a Wilton 352). Seal the top of the bag with a clip or rubber band.
Immediately pipe the roses once the meringue is colored. The meringue loses its structure as it sits, so you want to pipe it right away. Start in the center, then slowly pipe a swirl, working your way out from the center (see pictures above for reference). These don't spread as they bake, so they can be piped about 1 inch apart.
Bake both trays in your preheated oven for 1 hour and 15 minutes, then turn the oven off and let the meringue sit in the oven for another hour. You can also leave them in the oven overnight once you turn the oven off (I usually do this and find I have the best results this way. Letting them gradually cool helps prevent them from cracking. At this point, the meringue should be fully cooked and look and feel dry to the touch. If they still seem wet or moist, leave them in the oven for another hour.
Remove the tray from the oven. They should have a crunchy exterior and a light, airy center. Let the meringues cool fully to room temperature, then enjoy! These can be stored at room temperature in an airtight container for up to a week. Avoid refrigerating them! It will make them soft and sticky.