Okay, I'm still new to the world of air brushing, but this is what I've learned so far.
You want to spray your lightest color first, which in this case would be the light sky blue on the bottom of the plane. You'll want to then use a "soft mask" by using something like silly putty rolled into strips. That should keep you from getting a solid line.
Browns are darker on the color chart compared with greens, but I'd go for an olive drab overall on the top of the plane. Most of these planes were issued in a dark olive drab from the factories and it's doubtful the AVG mechanics would care to repaint a different tone of green over the plane when supplies were very hard to get. It's more likely they would trade supplies with the British squadrons in the area.
Which then leads up to your next and final color, the brown. I'm not sure what the official paint code for the color should be, but I would assume it would vary due to local trading partners. Which is why some seemed tan, and others a more medium tone of brown. So honestly I think you can wing it with the brown.
I would suggest free hand spraying of the brown pattern, but your AB isn't adjustable. You'll need to try another soft mask and that should lend you more accuracy in replicating the pattern the way you want it, with more control.
Don't worry about pre shading as it will most likely get lost under the layers of paint and show up only in the bottom color. Which could cause you a bit of grief trying to match up the post shading after the top color is on.
Once you're happy with that and all the paint is dry and unmasked give it a good turn in your hands and admire your handiwork. Let it cure a bit and patch up any last minute problems or minor scratches or overspray leaks from your masks.
Next you'll want to clear cote it with a nice gloss and get your decals in place. Make sure your stars are pointing up lol. After the decals are cured spray another clear over them to seal them in.
Now this is where you can do a couple of different things. Dot filters, panel washes, dirt washes, pigments, and oil washes. This is the part where you pull the little kid inside you out of the dry storage and let him play with finger paints, metaphorically speaking. You're basically painting over your painting as if it were another layer in Photoshop. If you mess something up, you can erase it, start over and get it the way you want it without destroying the work you've already done.
When the kid inside is beaming with pride you seal it all up with a dull cote or two. Unmask your canopy and finish off any last minute details such as antenna wires and such.
That sounds like a lot of work, but once you get into doing things regularly you'll start trying different ways and effects and such. It's quite enjoyable to experiment lol.
Sorry for the horribly long post and my lunatic ramblings and bad puns. I hope some of this is helpful and that I'm not just reiterating the rhetoric you're already familiar with lol.
Jetmaker, I fully agree with you. You can tell he's enjoying it very much and that he's doing a very fine job of this kit. I really like the attitude and enthusiasm :D I am enjoying this thread lol.
Edit; I almost forgot to mention something!
If you get the olive drab on and later decide it's not, say, green enough then you can brush on a filter to change the hue of the color. Same goes for the brown if you think it needs more yellow tones or a more faded tone to it. For a more faded tone I'd go with a light tan filter and let it change the hues to a lighter more slightly faded look. I may try that myself.
Incase you're not familiar with making filters I will give a short description, but if you are familiar with them then kindly disregard this next bit. :)
A filter is going to be a very diluted paint. You want it thinner than a wash would be and something close to a light tea in consistancy. Simply brush it over your color, but don't let it pool in the cracks like you do with a wash. you just want to lay this thin filter over the surface to change the tone of the color under it. Depending on the color of your filter it may take several layers to get the effect you want, but a little inconsistency in how thick the layers are can really work to your advantage in realism.
And you can use a filter on anything, including figures. Say you want the pilots vest to be just a litle more orange. Make a filter and change the tone. Done.
Once again, I hope that is not something you've already learned and I'm just wasting your time having to read all of this. :/
I hope to see your next pictures soon. :) Best of luck, and have fun with it!