Position the first layer of cake in the centre of a cake turntable and ice the top of it with white icing using a straight spatula. Repeat with the second and third layers of cake.
Ice the sides
Using upward strokes with a straight spatula, roughly ice the sides of the cake with white icing.
Colour additional icing
Separate the rest of your icing in half. Leave one half white and then separate the other half again. Using food colouring, tint one of the quarters lightly and the other quarter a little darker.
Ice the top of the cake
Using a straight spatula and white icing, ice the top of the cake, smoothing the icing carefully.
Pipe on darkest icing
Fit an icing bag with an extra-large round tip and fill the bag with your darkest icing. Pipe the icing on to the base of the cake evenly, slowly turning the cake turntable as needed. Repeat for two additional rows on top of the first.
Pipe on lightly coloured icing
Repeat with your lightly coloured icing.
Pipe on white icing
Repeat with white icing until you reach the top of the cake.
Using a straight spatula, smooth the icing along the sides of the cake while slowly turning the cake turntable.
Using an icing smoother and small upward strokes with the straight spatula, gently blend the colours while slowly turning the cake turntable.
Smooth top edge
Using a straight spatula, smooth the top edge of the icing.