How to et effects on exaust nozzles

I have had good results by airbrushing the entire exhaust with gun-metal (Testors) and once it is dry, going over that with a low pressure airbrush using burnt metal metalizer (Model Master) on just the tips, this allows the burnt metal to be prevalent at the tips, and fading into a darker gun metal near the base of the exhaust. It also has the added effect of popping out some of the details on the larger nozzles (specifically the turkey feather details on the 1/32 F-15-E from Tamiya)
Hope this helps!

I've used watercolor paint(the kind in the little sets-a bunch of little paint "pools"). Wet a cotton swab and rub it into the paint; blue on aluminum paint works good here. Also, use various shades of pastel chalk. Blend everything together with swabs. Gary