The airbrush way is probably the easiest, provided you have good control of your tool. I'd mix the camouflage paint with another colour to make it look lighter (remember green gets lighter with yellow), or faded (in this case mix with white). Use the airbrush to 'dust' the paint over the areas you want faded. Do not stop in any one place, keep moving even if in circular motion over a particular panel. The centre of the panel should receive more paint than towards the edges.
The oil/brush method is probably more forgiving. Make sure your paint, if enamel, is really dry (several days!). Acrylic base paint will be fine. use Cadmium yellow and/or white. Place little dots of paint in the centre of the panels you want to see 'faded' then take a soft, flat, clean brush and slowly spread this little amount of oil paint into the base color. Oils do not cover well, they are rather transparent, but using lighter shades will leave a distinct mark on your model. The trick is to blend the oils so well that you can not see a stark demarcation between the lighter central area of the panel and the edges. As you start spreading the paint, use a clean rag to remove the excess paint from the brush and keep spreading, again in circular motion. Do not clean the brush with thinners until you have completed the process!
I forgot to mention that I'd apply my washes beforehand. The final drybrushing of edges and engraved details can be done later, again, if you want, using oils (they are more forgiving than acrylics, that dry way too fast, or even enamels.
A word of caution, though, oils can take several DAYS to dry (that's normal), so beware where you put those fingers!!!!!