I always seem to have ready to roll icing in my cupboard, an alternative would be a coat or two of royal icing - once it was set it could be decorated in the same way...so many possibilities!
300g Leftover cake
Dollop of buttercream
1 tbsp Oil
1 tbsp Jam (lump free)
1-2 tbsp Icing sugar
200g White ready to roll icing
Place the leftover cake into a large bowl of a food mixer, mix until it is reduced to crumbs.
Add a small dollop of buttercream, adding more if needed.
Mix on low until the mix has a texture of wet sand.
Grease a silicone mould with the oil.
Press the cake mix into the moulds or the eggs can be shaped by hand if preferred. Chill in the fridge (or freezer for 30 minutes) for an an hour or so until the mix is firm (ish). I found that it never really sets firm but that depends on the amount of buttercream added.
When the eggs are firm enough, turn out and brush with the warmed lump free jam.
Roll out the ready to roll icing on a surface lightly dusted with icing sugar. Cover each egg with the white icing.
Smooth the edges and trim off the excess icing.
Don your latex gloves and protect your work surface with a some baking paper. Dip a new toothbrush into food colouring, run your finger over the bristles and sprinkle the eggs with your chosen colour.
Wash the brush between each colour and repeat until the eggs have a little of each colour. Place the speckled eggs into the paper cases and allow to chill in the fridge until needed.