Hey Stan
If you can get a hold of issue 009 of Tamiya Model Military International (Available from their site:
http://www.modelmilitary.com/ ) It has a really good introduction to using dot filters by Mig Jimenez.
The gist of the article is that lighter colours should be used closer to the panel lines and darker colours in the centre of panels. As to which colours to use, most colours are appropriate as long as they are likely to actually occur in real life. It would help to consider which panels would have which stains, for example panels near jerrycans or fuel caps would have darker black/blue/brown stains.
Hope this helps
Anirudh