Bake for about 30 – 35 minutes until risen and golden brown. Set aside to cool completely.
Once cooled enough, cut the cake into a semi-circular and flip it over upside down onto a board.
Icing the cake:
Place the butter in a bowl and beat until it is very pale and soft.
Continue beating, gradually adding half the icing sugar.
Add the milk and the rest of the sugar until the icing is smooth and easy to spread.
Divide the butter icing into 8 bowls and add the respective food colouring: 1/2 tbsp royal blue mixed roughly to give a marbled effect, 1 tbsp royal blue and violet mixed together, 1 1/2 tbsps royal blue, 5 tbsps grass green, 1 1/2 tbsps yellow, 2 tbsps orange, 3 tbsps red.
Begin by flat icing the central semi-circle in the blue and white marbled icing.