Bish, Eric... first off thanks for taking a look and the nice comments.
Bish, if you look at a lot of the armor painting/weathering videos by guys like Mig Jimenez et al, a big part of what they talk about is getting tonal various in the colors. A lot of that is achieved with pigments and oils etc but in my mind that should all apply to aircraft too. A guy that used to post here is big on this technique but can't tell you if it's his idea or not. He calls it black basing. After you do the squiggly/mottled color the color coats are applied thinly to let some of that unevenness come through. Leaving the panels line a little less covered is sort like the standard pre-shade method which I think looks too straight and even. I am just starting out with this technique and it's tough learning went to stop the coats of paint. Very easy to over do the color coats and cover that up too much. My pictures conceal most of the effect but it is most obvious on the underside as Eric mentioned. But it's there is the upper sides too.
Here is doogs website on this technique and he has it down pat. The A4 Skyhawk at the top of the page is a perfect example of what can be achieved.