Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line 3x20cm round cake tins with baking paper.
Mix the cake mix, water, oil and eggs in a large bowl. Whisk by hand or electric mixer for 2 to 3 minutes until smooth and creamy.
Pour the cake mixture evenly between the 3 cake tins.
Bake in the centre of the oven for 18 to 20 minutes or until a rounded knife inserted into the centre of the cake comes out clean. Leave in the cake tins for 10 minutes. Remove from the tins and place on a cooling rack. Peel off the baking paper. Cool completely, about 30 minutes.
Spoon icing evenly between 3 separate small bowls. Stir in a different colour of food colouring into each bowl to desired shade of colour.
Fit piping bags with medium plain nozzles; fill each with a different colour icing. Place first cake layer onto a serving plate. Pipe around the edge of the cake using the green icing, forming a thick icing border. Pipe a little more in the centre and spread out using a spatula. Reserve remaining green icing for decorating top of the cake. Top iced cake layer with the second cake and repeat border and filling with purple icing reserving remaining purple icing for top. Place the third cake on top and repeat with the orange icing, spreading out fully on top of cake reserving some orange icing for mounds. Pipe remaining icing into mounds over the top of the cake, alternating colours and sizes of mounds.
Decorate as desired using sugar eyes on icing mounds and sides of cake. (If using white chocolate drops and icing pen, use icing pen to make a small circle on the flat side of each white chocolate drop to create an "eye". Place drops flat side up onto icing mounds and sides of cake as desired.) Store covered at room temperature.