definitely not an expert... and I will have to look at the LHS or maybe michaels.,.. there is a "rubbing compound" type stuff that burnishes a material a certain metallic color... my mom used to use it on ceramics and plastics with her crafts...
That with a light bluish, then a light marinish/aqua wash would give that same effect I think... with a little red up near the origin of the exhaust just because the rub is kinda waterproof whch would produce the 'mottled' effect that exhaust has...
Like I said someone more experienced might be able to answer this better, but I have seen the same effect done on other materials, just need to check tomorrow to see if it will work on model plastic...