I’ve tried a few combinations - no pics sorry.
For a rusted effect, I’ll start with a base spray, something like humbrol’s gunmetal. This isn’t very metallic and has a pitted surface. That’s not great for a natural metal finish required elsewhere, but for a rusty exhaust it works really well. The pitted surface allows weathering powders - I use dust from sanded pastel sticks in a variety of rusty shades as well as dark grey/black for the sooty end.
Another method sees a base coat of grey primer spray, a thin layer of matt black, giving the same pitted effect. The metal tones are achieved by buffing with the brush I use for polishing up mr metal paints - there is usually some residual matter on the brush and this gives the glint to the high points between the pits, then weather there as above.
For cleaner new exhausts I brush on and buff mr metal, iron or dark iron colour, which is more difficult to weather.