Preheat oven to 350°F (325°F for dark or nonstick pans).

Place paper baking cups in each of 24 regular-size muffin cups.

In a small bowl, mix 1 1/2 cups crushed pretzels, 2 tablespoons sugar, and 1/4 cup melted butter or margarine until blended.

Spoon about 1 tablespoon of the pretzel mixture into each muffin cup.

In a large bowl, beat cake mix, margarita mix, water, vegetable oil, egg whites, and 3 teaspoons lime zest with an electric mixer on low speed for 30 seconds, then on medium speed for 2 minutes, scraping the bowl occasionally.

Divide the batter evenly among the muffin cups (two-thirds full).

Bake for 18 to 23 minutes, or until a toothpick inserted in the center comes out clean.

Cool for 10 minutes; remove from the pan to a cooling rack and cool completely.

In a medium bowl, fold together the whipped topping, key lime yogurt, and 2 teaspoons of lime zest until blended.

Add green food coloring if desired.

Frost the cupcakes with the yogurt mixture.

Sprinkle the remaining 1/2 cup of coarsely crushed pretzels on top of the frosted cupcakes.

Store loosely covered in the refrigerator.