Preheat your oven to 200°C. Lightly grease a 9" x 5" loaf pan or a bandit pan.
In a bowl, whisk together the flour, and baking powder, set aside. In another large bowl, beat the butter or margarine and sugar until very light and fluffy.
Beat in the eggs, one by one. Scrape the bottom and sides of the bowl, and beat until the mixture is very light and fluffy. Add essence and lemon zest and beat together.
Fold in the flour and baking powder.
Divide the batter into three equal parts. On each butter, add the food colour and thoroughly mix till well incorporated. If using a bandit pan, start with red batter, top with yellow batter and finally green batter. If using a loaf pan, start with green, top with yellow and red would be last.
Bake the cake for 35 to 40 minutes, until it springs back when pressed lightly in on top, and a long toothpick inserted into the center comes out clean.
Remove the cake from the oven, and loosen its edges. Wait 5 minutes, then carefully turn it out of the pan onto a rack to cool. Serve or store for a day or two before serving. Wrap well and freeze for 3 months storage.