Jet exhausts stains

I havent done this technique yet. But I have often thought that using a clearcoat that is tinted with different colors (a clear bluish color, a clear goldish color, etc) would look good sprayed over an already painted  tailpipe. I havent tried this on a model or with model paint. However, I used to do airbrushing on motorcycles and helmets ( with high end automotive paint) and this was one of my go-to techniques for shading and it worked great.

It depends a lot on what the tailpipe material is- different mfgs use different kinds of metal on the tailpipe.  Some merely dull down, others take a fancy multicolored "oxidization" hew.  In the first case merely picking the right color does it.  For the second type I find the best way is to use an airbrush with very thin paint and flow dialed way back (double action brush is best for this but careful adjustment of SA brush will work too).  You need to study actual photographs to see what the shades of the coatings are.  This is similar to burned chrome on headers of race cars and cycles, where the chrome takes on thin layers of a rose red and purple-blue.