Fill a saucepan with 1 inch of water. Bring to a simmer over medium-low heat. In a clean bowl combine egg whites and Splenda Granulated Sweetener. Place bowl over simmering water and whisk mixture constantly for 5 minutes, or until Splenda Granulated Sweetener is completely dissolved.
In a separate bowl add margarine to the bowl of a stand mixer and beat until light and fluffy. Set aside.
Transfer egg white and sweetener mixture to stand mixer and beat on high for 10 minutes until stiff peaks form. Lower speed to medium and mix in margarine 1 tablespoon at a time. Add salt and vanilla and continue mixing for 3-5 minutes or until frosting is smooth and fluffy.
Add gel food coloring if desired and mix until food coloring is completely incorporated.