To create your grey icing for the detail, mix together icing sugar and water in a bowl until you get a smooth paste. You're looking for a consistency which runs off the back of the spoon like a really thick custard. If you've added too much water, bring it back by adding more icing sugar. Similarly, if your mixture is too thick, add a teaspoon of water at a time until the desired consistency has been met.
Add a drop of black food gel colouring to make a light grey colour overall.