First I apply the paintscheme so you can really see how the weathering works on the paintscheme. How it works out really depends on the colors used.
Then I look for images like these:
http://users.skynet.be/sky38711/allf...orkima/wk2.jpg
http://www.german-helicopter.com/sta...ing_static.jpg
http://www.raf.mod.uk/downloads/wall...ng_01_1280.jpg
http://www.ogwen-rescue.org.uk/visit...by_seaking.jpg
http://www.bharat-rakshak.com/NAVY/Images/Seaking18.jpg
http://www.bharat-rakshak.com/NAVY/Images/Seaking12.jpg
Note the exhaustdirt from the different angles and how it influences what a color looks like