Combine butter and cream cheese in a bowl and mix with mixer until light and fluffy.
Add in your lime juice and green food coloring until desired shade is reached (remember it will lighten slightly when you add your powdered sugar).
Add in your powdered sugar, one cup at a time, until the desired consistency has been reached (could be slightly less or more than five cups).
Frost each cupcake as desired. You can pipe on a ring of frosting and sprinkle with sugar and salt mixture, then finish frosting your cupcake, or frost your entire cupcake and sprinkle the sugar and salt mixture on top.
Due to the cream cheese in the frosting, it's best to store these in the refrigerator.