Google seems to indicate that warplock bronze is a copper colour, not rust. You need to go for shades of brown and orange. Here are some images for reference:
There are a couple of different ways of achieving this. Personally I use powdered pigments as they give the metal a nice dusty look- you can make pigments by sanding down coloured chalk pastels to a paste, which are cheaper than the pigments you often see in modelling shops.
Another way to do it is to make a varied rust effect the base coat of the metal areas: stipple on different shades of rust colour using pieces of torn up sponge that you get in blister packs (or used to, anyway). Then dry brush metallic paint over the top.
I don't know modern GW colours, but what used to be "dark flesh" is a nice brown with red in it. That might be the darkest shade, and adding more orange and yellow to the base of dark brown. Then a dry brush of a lighter silver colour- used to be called chainmail back in the day.
Lastly you could use the relevant rust colours as washes over a metal base, and dry brush over the top afterwards to bring out the detail and remind people that they're actually looking at metal.
Googling "miniatures rust tutorial" will bring up a few pages of helpful results with a lot of different techniques like the ones I've mentioned above, with pictures and step by step guides.
Hope that helps.
