Preheat oven to 160°C. Grease and line the base and sides of three 20cm (base measurement) pans with baking paper.
Use an electric mixer to beat 125g butter, ¾ cup (165g) sugar and ¼ cup of the blueberry jelly crystals in a bowl until pale and creamy. Add 2 eggs, 1 at a time, beating well after each addition. Add 1½ cups (225g) flour, 2 tsp baking powder and ½ cup (125ml) buttermilk, in alternating batches, stirring well after each addition. Spoon into 1 prepared pan and smooth the surface.
Repeat twice with remaining butter, sugar, eggs, flour, baking powder and buttermilk, using lime jelly crystals for the second mixture and raspberry jelly crystals for the third mixture.
Bake for 1 hour or until a skewer inserted in centres comes out clean. Set aside in the pans for 5 mins to cool slightly before turning onto a wire rack to cool completely.
To make the icing, use an electric mixer to beat the butter in a bowl until very pale. Add the icing sugar, in batches, beating until well combined and creamy. Divide into 3 even portions. Stir remaining blueberry jelly crystals and 1 tbs of milk in a bowl until jelly dissolves. Tint 1 icing portion with a little jelly mixture and blue food colouring to make light blue. Repeat twice with remaining icing portions, remaining lime and raspberry jelly crystals, remaining milk and green and red food colouring.
Use a large serrated knife to level the top of the blue cake. Spread the top with blue icing. Repeat with the remaining cakes and icing. Carefully layer the green and pink cakes on top of the blue cake. Use a flat, wide knife to spread the remaining pink, green and blue icing evenly around the side of the cake. Decorate with sprinkles and bunny chocolates.