For the gun smudges, try making a new layer. Using the airbrush, set it to a low flow 15-20% and low opacity 15-20%. You can change these settings by double clicking on the tool button in the GIMP menu.
Slowly add some shading around the back portions of the guns. Go to the filter menu and go filters/distorts/wind. This filter will add a wind blown effect to the smudges. Works like a champ. You can use it for hydraulic leaks, fuel leaks--all kinds of stuff. Just make the sure the wind blows in the right direction.
As for the colors, there's a couple of lists around that have the RGB values for certain USAAC colors. If you can't find them online, I have some I can look up for you. In particular:
OD Green 60, 55, 39
Medium Gray 128, 129, 135
With these two colors, you can paint just about everything pre 1944...
You can use the path tool to draw the rivet lines, then stroke the line with a dotted pattern that will be the rivets.
Also, if you haven't already, there's a series of tutorials at
http://www.gimp.org that can help you learn the interface and how the tools work.