I haven't done enough armour, or inspected up close, to comment on casting marks. But I have tried drybrush as Kevin mentioned - both lighter and darker than the base colour, sometimes going back in with the base colour - I liked the slight stipple effect of rough mettle you can get. If you are afraid to try it on your model - grab a scrap or 'extra' from the shelf and try stuff on that! I agree, this stage can be very fun, especially if you know there is a path to 'undo'.