Preheat oven to 200º F. Line three baking sheets with parchment paper. Trace the cactus cutter (or pattern) six times on each piece of parchment paper. Return the paper to the baking sheets ink-side down and set aside.

Add egg whites and sugar to the bowl of a standing electric mixer. Whisk to combine. Set the 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 the mixture 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, zest the limes into a small bowl. Once finished, slice one of the limes in half and squeeze 2 teaspoons of the juice into the bowl with the zest. Mix well and mash with the side of a silicone spatula.

Once the meringue has cooled, press the lime solids to one side of the bowl. Reduce mixer speed and pour the liquid into the meringue (it's okay if a few stray bits of zest pour in with the liquid). Add food coloring, if using. Whisk on high speed until evenly combined.

Fit a pastry bag with a large open star tip and fill the bag with the meringue. Add a bit of the remaining meringue on the spatula or whisk to the four corners of each baking sheet underneath the parchment paper. Smooth the paper over the meringue to keep it in place during piping.

Begin by piping the arms of each cactus, starting at the top of each one and releasing the pressure as you finish piping just inside the area where you will pipe the central stem.

For the stems, start at the top and pipe downward with gentle, even pressure. Be sure to pass over the end of each arm. As you near the bottom, release the pressure while pulling away from the bottom edge.

Use any remaining meringue to pipe barrel cacti in the open areas on the parchment paper. Hold the tip straight down while you pipe small mounds, gently releasing pressure and lifting up as you finish each one.

Sprinkle white nonpareils over the freshly-piped meringue and gently press icing flowers into the cacti.

Bake the meringues in the 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.