Preheat the oven to 350 degrees Farenheit. Grease and flour three 9-inch round cake pans.
In a large mixing bowl, mix the sugar, flour, salt, baking powder, baking soda, and gelatin, and mix well.
In a medium sized bowl, mix the eggs, oil, orange juice, lime juice, lemon juice, and vanilla extract. Add the egg mixture to the dry mixture and mix until just incorporated.
Divide the batter evenly among the 3 pans and bake for 35 to 40 minutes, or until you can insert a toothpick and it comes out clean.
Cool the layers in the pans for 5 minutes, then turn them out onto cooling racks and allow them to cool completely.
In a mixer, cream the butter and cream cheese. Add confectioner's sugar, lime juice, vanilla, and food coloring, and mix until the icing is smooth and easy to spread. Spread the icing between the layers and on the top and sides of the cake. Sprinkle the top with the lime zest.