Sundog, can't wait to see the S-5 you're working on! About metal, I think the real trick is not too blue, not too anything, but just enough that it's not full gray. I usually go for a brownish/yellowish/blueish mix and try to get a variation of these colors over the surface.
***BUT NOW, is it done yet?
(Maybe the first finished profile of the series?)

I'm not sure if I've got enough weathering on it and I might need to bump up the fabric detail on the rudder, it kind of gets lost in all that green stuff
Here's the real plane in case you'd like to see it-