Preheat the oven to 180°C (160°C for fan assisted ovens)/ Gas Mark 4. Line 2 muffin trays with paper cases.
Mix the eggs, oil, water and cake mix gently together and whisk (by hand or electric mixer) for 2-3 minutes, until smooth and creamy.
Spoon the cake mixture into the paper cases, to just over half full.
Bake in the oven for 18-22 minutes, until golden and risen. Remove from the oven and leave to cool on a wire rack. Cool completely.
Place half a tub of icing into 4 small bowls and stir enough food colouring into each to create blue, green, red and yellow icings.
Spread a heaped tablespoon of icing onto each cupcake and smooth down with a spoon. Arrange the sugar coated chocolate beans in different colours to create scales, the banana foam for a tail, the chocolate heart for the fin, the half marshmallow for an eye and the cheerio for the mouth. Finish off by adding a pupil to each marshmallow with the writing icing pen.