Preheat oven to 350° F. Lightly spray a 13×9-inch pan with cooking spray.
In a large bowl, mix butter and ½ cup powdered sugar together with an electric mixer. Mix in 1¾ cup of the flour on low speed, just until well combined. Press in pan. Bake 22 to 25 minutes or until set and lightly browned.
Meanwhile, in a large bowl, whisk together eggs and sugar. Add remaining ¼ cup flour and salt, mix with a whisk until blended. Add in lime zest, lime juice, food coloring, and milk and whisk until smooth. Stir in tequila.
Pour the lime mixture over partially baked crust. Bake 25 to 27 minutes longer or until center is set.
Cool completely and then chill in the fridge until ready to serve. Sprinkle with powdered sugar and cut into bars. Store bars tightly covered in the fridge.
If desired, omit mint leaves and substitute ½ teaspoon mint extract. Add extract with the rum as directed.