it is kind of hard to critique your work with a picture that size, and mind you I am being extra critical because you asked us to be (because it actually looks pretty good to me). With those qualifies in mind...
{opinion}
First off, I tend to think pre-shading and panel accents are overdone in many cases. It is a nice artistic effect to exaggerate, but it is not particularly realist when more than lightly suggested in most cases (granted there are exceptions). But then this just goes the way of the panel line argument - I agree panel lines are exaggerated on models, but I still like to accent them some regardless, so I can appreciate the pre- and post-shading point of view as well, even if i don't fully subscribe to it.
{/opinion}
I'd say you probably have a basis to work from. Maybe the gray faded areas look a little too bright (but that might not be the case in real life - digital camera and monitors are famous for skewing color balance). And you faded panels sort of look blotchy - more like the fading has come from wear and scuffing rather than weather exposure.
I bet hitting it with a very dilute spray of some color like "buff" or a light khaki would go a long way to adding to the faded feel while simultaneously bringing your greens and grays together to make them look like they belong on the same object as well as giving a more overall weather/sun faded look.
I suggest spraying the fading color greatly thinned (at least 2:1 thinner to paint, maybe more) and with fairly high pressure from about a foot away from your model. only hit the upper surfaces up the wings, tail, and fuselage (don't include the prop).
Be very cautious about how much you spray - a little goes a long way. do a light mist, let it dry, look at it for a while (and in a mirror - you'd be surprised how much that changes your perspective on your work), and decided if you want a little more or not. You are only trying to calm things down a little and reduce the tonal contrast between colors, not make a visible tint of the buff color.
If you look closely in the following photo, you can see the effect on one of my models:
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Fading spray probably better seen in this photo - compare the wing to the side of the fuselage:
(and yes, i did wind up changing the camo pattern on the upper wings)
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compare that to what it looked like before I did that:
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There was also a heavy overall wash earth brown acrylic paint between the last two pictures as well - some thing you might want to consider if you are looking for something really weather beaten - follow up later (after gloss coat, decals, and flat coat) with a darker wash (i like to use watercolors at this stage - burnt sienna with a touch of black) along the panel lines to restore the looks of your pre-shading since the dark earth wash will make your panel lines look like they do in my second picture.
feel free to use all, some, or none of my suggestions.
And be sure to experiment with that fading spray on something besides your model till you get the right effect.
like i said, I think you have a pretty good basis anyway from what i can see.