For the rainbow cake fish first prepare the dough. For the dough, beat eggs with sugar and vanilla sugar until foamy.
Mix flour and baking powder and add to the mixture alternately with the water/oil. Divide mixture into 4 containers and add food coloring to 3 of each.
Take a cake ring and put alternately with a big spoon different colored dough in the middle (runs dan by itself to the edge).
Bake the cake at 180 degrees top/bottom heat for about 45 minutes. Let it cool down, roll out the fondant and put it over the cake.
Press it down and cut away the edge. Now cut out a corner and place it at the back of the cake (so the pattern is nicely visible).
Now decorate as desired. Have fun and enjoy your meal!