Blondey, are you colouring up the icing just for the decorations or for covering the cake too? You can overwork icing but if it's for the decorations it doesn't matter, it's only when you're covering a whole cake that you nee to be careful about overworking. If you want the decorations to set hard leave them out on the side, open to the air. Also, once you've baked and iced your cake don't fridge it! It's not necessary and will make the icing wet and yucky x