Bronze is not the same colour as gold or brass and doesn't weather the same as either. Bronze, when freshly polished, is quite pinkish as you might be able to see if you surf the web looking for bronze foundry/casting information. As it oxidizes it tends towards reddish-browns, unlike brass which tends towards a greenish hue, or gold that of course doesn't tarnish.
Depending on the metallics you have available to you Marc for fresh bronze try a mix of gold, silver and Burnt Sienna. For a Celtic chieftain the bronze would almost certainly have been highly polished - just as with their clothing being the best and most expensive of what was available, their armour would be a reflection (no pun intended) of their status. For a medium tone the same colours with less silver and a little added Burnt Umber (not Raw Umber, it's the wrong hue).
An oxidised finish is very very unlikely for any item used for personal protection regardless of rank but if you wish to do this the colour bronze tends to turn naturally is a deep chocolate, so Burnt Umber is a good basic starting point. Brushing this over the lighter colours will probably give you the best effect, so you can lightly remove some of it in areas subject to wear - under the arms, where belts or other items of equipment rub against it - to show the polished colour.
If you don't have a good gold and silver Marc there is a very good Bronze colour in acrylics made by Daler-Rowney in their Cryla range which is finely pigmented and an excellent colour match, you could use this as a base and work on top of it in oils if you wish.
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