Preheat the oven to 170C/325F. Prepare a 9-in(22cm) tube pan that is ungreased.
To make the egg yolk batter, beat egg yolks with sugar till it is pale, then add in salt and oil, mix it briefly till it looks like mayonnaise.
Add in grated orange rind, orange juice and orange colouring (I omitted) and mix well. Fold in sieved flour until it forms the batter.
To make egg white foam, beat egg whites and cream of tartar until the mixture forms soft peaks. Gradually add in sugar, beating at high speed until stiff peaks form.
Gently fold the beaten egg white foam into egg yolk batter in 3 batches until fully combined. (Do not overmix)
Pour batter into an ungreased 9-in(22cm) tube pan. Tap the pan a few times on the table top to get rid of any trapped air bubbles in the batter.
Bake in a preheated oven at 170C/325F for 50-60 minutes or until cooked. (When lightly pressed, the cake should spring back)
Immediately upon removing the cake from the oven, invert the pan (turn upside down) and place on a bottle or flat surface so it is suspended over the counter. Let it cool completely before removing the cake from the pan.
To remove the cake from the pan, run a thin bladed knife or a palette knife around the side of the pan and it�s center core, release the cake and run the knife along the base of the pan to remove the cake.
