Great halls require hard metal, but even on the preview site for buildings you can see how other metals would look like (ie. silver or gold instead of hard metal). It would be very nice if we could use a sketchbook to change hard metal variable materials to any metal on an already done building.
If the issue is "WELL YOU CANT BUILD IT IN GAME SO IT SHOULDNT BE POSSIBLE WITH SKETCHBOOK" (not a good argument when sketches can be used to increase inventory space of containers) then allow us to make a plating (with gold/silver etc.) on buildings that would cost let's say 1/5th of bars required to make that building.