I have a theory about colour. It is that should you not know what to paint some model metalwork, use green.
Not any green but that particular shade of green favoured by engineering companies and municipal organisations. Sort of Brunswick green but perhaps a little lighter. Bit like British Racing green too. Did GB just buy a big pot of it in the 1940's a decide to use it all up I wonder ?
In support of this theory I present exhibit A, a peeling iron railing I spotted locally. Under the layers of modern black paint, what do we see ? Municipal Green.
I'm not sure how you'd replicate this paint finish on a model. At a guess I'd suggest you would paint the spear rust colour. Sprinkle some salt on this, paint it green. Add more salt. Then paint it black. Once dry, wash the salt off. If anyone tries this, please let me know !