hiya,
I love doing the exhausts on bikes, and this is my technique.
I use alclad II paints which are superb metallics paints to use as they blend in perfectly to whatever you are doing , in this case , the exhaust.
After priming the parts with grey primer and giving the parts a couple of coats of tamiya silver as a basecoat, I then give the exhaust a couple of coats of alclads "pale burnt metal" to dull the silver and give it a burnished look. This is followed by a coat or two of "jet exhaust " near the exhaust ports to highlight the burnt metal look. I will then airbrush a layer of thinned tamiya clear blue and/or clear orange to burnish out any superheated areas even more. The result:
