Ive never used a colour for this kind of effect, perhaps you can buy and edible silver paint or powder? If not, to get a silvery affect on my cakes, i use a white fondant base then build up layers of silver/grey/gun metal/white lustre dusts.
Perhaps you could get a gun metal edible powder on the fondant before dusting....maybe leave to dry and cover with confectioners glaze to give the sword shine.
Hope this maybe helps, good luck! Let me know how it goes