Don't use varnish to clear over your decals or paintwork!!
The stuff IS yellow, or WILL yellow over time, giving all white areas a distict yellow tint.
Use Future. Even with numerous applications, it still a better option than varnish.
I've actually been in the pits at the Australian 500cc Grand Prix (the Kenny Roberts Modenas pit) and in wandering around and having a look at many of the teams found out that most of the markings are vinyl decals applied over the painted fairings, and there are distinct differences in the surface finish between them. Some decals are matt, some semi gloss, and some gloss. There was no clearcoat over any of these decals, as they must be periodically changed from race to race (some countries don't allow tobbacco sponsorship and those decals must be removed for the race or swapping teammates fairings ) or sponsorship deals change and their markings are removed or just moved - it'd be hard to replicate this effect in 1/12 scale, so I just use Future over the decalled fairing.
The exhausts are a different matter - have a look at the individual team website - I go to http://motogp.tiscali.com/en/motogp/index.htm
and from there to Riders/Teams and that page will have a link to that teams own website - they always have excellent photos of their bikes. Also do a search on Google for that team which should provide links to other images.
I use Metalizers for overall color, have used clear yellow and clear blue for the heat affected zones (the curves in the pipes as well as the welds) - but try to maintain a high degree of overall cleanliness - remember these are million dollar prototype motorcycles and incredibly well maintained - there's no such thing as a dirty GP bike. A clear with Future or Satincote works well for me.
Good Luck
LeeTree