Help!!!

Well, I finally figured it out after much trial and error :)

It turns out it was a tuffer problem:

1. Not enough thinner - as per Phil's advice, 2:1 thinner to paint (or even a bit more thinner) is better. Although even when thinning it to such consistency I still had overspray, so:

2. Not enough pressure - I got really frustrated and cranked the air compressor to the max. According to the gauge on the AC, my max pressure is 60PSI, although once I start using the airbrush the pressure quickly drops to about 25PSI. Now I am really quiestioning the accuracy of that gauge, so maybe this was the culprit.

In summary, for those folks in the future that may run into overspray problem like I did - just thin the paint a lot and increase the pressure as much as you can. Maybe it would work for you too :)

As per Gerald's reply above, normally one would start with the lightest colour, in this case the dark yellow, as dark colours cover light colours better than vice versa.

I do think that there is a similar amount of overspray with the red-brown over the green, it's just less noticeable because there is a lower contrast between those colours.

Can you be a little more specific about how much thinner you're using, the air pressure you're using and what the distance is between the surface and the airbrush?  Personally I would be going very thin (in the region of about 2 parts thinner to one part paint), with low pressure (about 15 PSI)  and painting very close (ie. less than 2 inches - probably closer to 1"-1.5")

Don't try to create the colour bands in a single pass - the distance required to do a swathe of colour of a sufficient width in one pass will guarantee overspray. Get up close and personal, outline and back-fill.

Well, I cannot visually see anything that could be wrong with the airbrush, plus it painted the red-brown just fine, so I can rule that one out. It's not dirty, since I clean it extensively after every color and I do a complete teardown after every third model.

Also, after I saw how it came out on the model, I did all kinds of experiments to see if I'm doing something wrong, including adjusting the PSI from 5PSI upwards in 5PSI incrememnts, painting from very short distance, thinning the paint gradually to almost runny consistency but it always had this spatter on the side. Now that someone mentioned, maybe the paint went bad? I have the same issue with a dark green Model Master I picked up from Hobby Lobby - those probably stay on the shelves for ever, but this one was ordered directly from Tamiya USA...

Bummer, since I was hoping for a simple solution, because of something I overlooked or did not know about...

First work colors from light to dark. Your problem appears to me that you didn't thin consistently as you did with your other colors. Also make sure your airbrush is clean including the cap/nozzle cover. Also work from the same distance as you did with any secondary or subsequent color. That way your line width will be consistent as well.

Practice, practice, practice. In this case, you might try working closer to the model to minimize the overspray you're getting (which may also require a bit less pressure).

Could there be debris in the airbrush tip causing the paint to scatter like that? Try a complete tear down of the airbrush and extensive cleaning. Inspect the needle and tip for damage, replace if there is anything visibly imperfect with either.

Once this is finished spray on something scrap. If making changes only do one at a time and write down adjustments / results in a log book.

The paint itself may be the culprit so if nothing remedies the situation try a different jar or brand.