Line 2 fairy cake tins with paper cases. Divide the batter evenly between them (make sure each is only just over half full, as you need space for the icing to set flat on top).
Divide the icing sugar between 2 bowls. Very gradually add a little orange juice to each until it is thick enough to coat the back of a spoon. Add a very little purple food colour to 1 bowl and blend it in well until you achieve the desired shade. Cover the bowl with clingfilm (the icing dries out very quickly) while you colour the other bowl in the same way, using the green food colour.
To ice the cakes, replace them in the fairy cake tins. This makes it much easier as the tins will hold the shapes of the cakes. Spread enough icing on to the cakes - using each colour on about half the batch - so that it is almost level with the top of the case, gently easing it to the edges with the back of a spoon. Allow to dry for a couple of hours. The cakes can be iced 2 days ahead, but add the flowers on the day they are to be eaten.
To finish, arrange the crystallised flowers and leaves on the cakes, using dabs of royal icing to hold them in place. Display on cake stands or plates, scattering any remaining flowers and leaves in between.