Add a little green patina — a very light coat of flat green (or a wash of the same).
Now let’s get into phosphor bronze. For those members who are saying “phosphor bronze? whatzat?”, phosphor bronze is an alloy of 90%-ish copper, 3.5 - 10% tin, and <1% phosphorus. This stuff has low susceptibility to chemical corrosion in a saltwater environment, a low coefficient of friction, tough (machinability rating = 20 vs. free cutting brass = 100), and resistant to mechanical fatigue (all which makes a preferred material for a ship’s propellers). It has a somewhat odd, albeit pleasant, color:
The phosphorus improves the fluidity of the molten metal, thus cleans up the grain boundaries, improving mechanical properties.
And this is what it looks with a little corrosion:
80,000 pound propeller from the U.S. flag merchant tanker, “American Osprey”.
First, paint the part completely with copper color. Then, paint with a thin, semi-transparent layer of gloss dark or medium brown. Some of the copper color should show through.
Then, a very thin coat, just a tint of a flat green- maybe olive drab even. This last can be applied as a wash, even, and should not be an even coat, but varied here and there. This is the patina coat.