There are many different ways to do that type of thing. Here are a couple of my faves:
Use different mixes of the base paint. Mix in a bit of white or other light color to lighten the OD.
There are also many different OD's out there on the market. Find a couple that complement each other and use them together.
With either of these two methods, start off with the darkest color. Then gradually add light coats of the lighter colors until you get the results that you like.
Some of the early OD used on B-17's took on a slight purplish haze when they faded. That's another avenue you may wish to explore, again mixing colors.
The bit about accentuating panel lines is my forte, actually, although I generally use a mixture of pastels and washes. The "pre-shading" technique mentioned by NWilliams is a very popular method. I've yet to really give it a serious go myself, but I've seen stunning results had by it.
Fool around on some scrap pieces or whatever you may have laying around. Try out a bunch of techniques, combine them, modify them to suit yourself and see what you get and which you like.
Fade to Black...