Looking good Vic. You're on the right lines with the paint mixes. That's the sort of thing I normally do, and also, I add a spot of white, and sometimes grey, to vary the tones even more. The use of gloss white/grey gives a different effect to using matt white/ grey, and some really subtle effects can be created. Also, if some areas are given a clear coat, and others left as painted, the effects are even more varied.
If you can get some, try using SNJ powder, in the Silver variety, to polish the model before painting. See the 'Bare Metal Made Easy' guide I produced some time back, I think it's on Page 6 of the painting questions thingy.