I got this idea from some blog or other, but could not find it again. (Let me know, if you have an example). I used it on a multi-melta.
Using inks/washes or possibly very thin paint build up some rings. Try to vary the intensity, so the colours blend. If it comes out a bit strong, it can be knocked back a touch by drybrushing with the base metal colour again. Update: Thanks to Siph, here's my reference.
