Preheat oven to 350F. Line cupcake pans with paper liners.
Combine flour, sugar, baking powder, salt, shortening, milk and vanilla in a large mixing bowl. Mix at low speed for 2 minutes. Scrape bowl. Add egg whites and additional whites or yolks, and mix at high speed until fluffy and smooth, approximately 2 minutes.
Fill liners 1/2 to 2/3 full of batter. Do not overfill. Bake 20-25 minutes or until toothpick inserted in center comes out clean. Let cool 10 minutes in pans then remove from pan, and place on wire racks to cool completely
To decorate the cupcakes: Center a cupcake on a turntable.
Take the green buttercream bag in hand and, while turning the turntable, apply steady pressure to pipe a border around the edge of the cupcake.
Switch to the red buttercream bag, pipe a rose, and transfer it to the top of the cupcake.
Note: Unless you have two rose tips, you will need to set up and fill the red bag after using the green bag. Be sure to clean and dry the tip before dropping it into the bag that will hold the red buttercream.
Remove the cupcake from the turntable and repeat with the remaining cupcakes.