Preheat the oven to 350°F, 177°C, or gas mark 4. Grease the Bundt pan and set aside.
In a mixing bowl whisk the flour, baking powder, and salt together and set aside.
In a stand mixer with paddle attachment beat the butter and sugar on medium high speed until the mixture becomes very pale.
Add the egg whites one at a time allowing 1 minute between each egg white. Add the sour cream and vanilla and beat until mixed.
Reduce the speed to low and add half of the flour mixture mixing until just incorporated, add all of the cream and mix until it is just combined followed by the remaining flour mixture.
Remove half of the batter and divide the remining half in half again. Color one of the quarters red and the other quarter of the batter blue. In the Bundt pan add alternating dollops of the red, white and blue batter. Alternate where the colored batter is placed to obtain the swirls later.
Bake for 40 minutes or until a toothpick or knife comes out clean.
Allow to cool in the pan for 30 minutes and then turn out onto a cooling rack for another 30 minutes or until the cake is room temperature.
Making the icing
In a stand mixture with the paddle attachment beat the butter on medium high speed. Reduce the speed to low and add the powdered sugar. Once all of the sugar has been incorporated increase the speed to high and beat for at least a minute.
Reduce the speed to low adding the milk and lemon juice. Increase the speed to high again and beat for another minute.
Color parts of the icing if desired and pipe the icing onto the cooled cake.