In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar, salt and lemon zest until light and fluffy.
In a small bowl, whisk together the eggs and vanilla. Slowly add to the butter mixture with the mixer on medium-high, scraping down the sides of the bowl after each addition.
Whisk the flour and cornstarch together in a bowl. Add to the butter mixture and stir on low just to combine. Add your food coloring at this point if you desire. Be careful not to over mix the batter.
Transfer batter to a pastry bag fitted with the desired tip. I used an Ateco #802 star tip for the green, an Ateco #801 round tip for the white and an Ateco #800 round tip for the red.