If you've selected a colour of leather for your book cover but feel the need to make it 'extra' (because face it, you need it to bump up your charisma stats) then a splash of gold, emerald or sapphire will certainly do the trick.
The Metallic Touch is a random spatter of gold paint added across the leather by hand. It's pattern is unpredictable, so there's no telling how the leather will look when it's fully build into a book cover. How's that for unique?
You can add any of the Metallic Touches to your cart if you've added a book cover or notebook to your cart as well. This can be an engraved or a fully painted version. In the case of the fully painted version, the Touch will be applied before the hand painted details come in.