Bronze is quite easy in fact ( alot easier than skin

)
Actually I cheat a little for this technique.
First I use real metal colors from Prince August but you can manage with any brand in fact. Apply a soft coat of bronze on your basing colour, mine is earth brown, then another one until your bronze is uniform and shiny.
Second go for a dark green wash all over it (Citadel wash will do he work perfectly), it is the colour of the corrosion on this particular metal, it will dull the shiny effect and create a beginning for the shadows. Alternatively for a more precious effect you can use turquoise.
Third step work the highlights with the original bronze colour.
Fourth step apply the final highlights with a very shiny gold, I use an airbrush colour, because the paint is very thin.
Last step work your shadows with a brown glaze.
You're done
PS - you can use the same process with gold just use a purple wash instead of green

cheers