The roses can be made a few weeks ahead. Divide the sugarpaste into 5 blocks (this quantity makes about 40 × 5cm roses).
When colouring the sugarpaste, the use of black is optional. It does make a more interesting colour, but use literally a pinprick when required. Mix all the 5 colours. Create a deep red with poppy red, egg yellow and liquorice black; a bright pink with claret, pink and liquorice black; a light pink with claret; a bright orange with poppy red and egg yellow; a lighter orange with primrose, egg yellow and liquorice black. Allow the sugarpaste to rest overnight (it will be much easier to mould).
Make 8 roses in each colour. Sprinkle with glitter and store in a covered box at room temperature.
When ready to assemble, place the cake on to the serving plate. Arrange the roses all over the top.
Finish by wrapping the ribbon around the cake, using a blob of extra Ganache at the back to hold it in place.