Working on the front landing gear bay of my F-4 phantom. Painted it white, and then tried a few things to make it dirty:
Tamiya black panel line wash - now it’s medium gray…
Citadel acrylic wash agrax earthshade - now it’s brown
Citadel acrylic wash nuln oil - this is much better than the Tamiya black panel line wash, but it’s still gray now rather than white.
So, what do you use to add dirt to the underside of cold war navy planes with insignia white underneath? After I figure it out for the landing gear bay, I’ll need to be able to do it to the whole underside without making it not look white anymore…
I’d want to test this idea on something that doesn’t matter first, but pastel dust (whatever colour you want) or maybe just charcoal might do the job? Scrape a stick with the flat of a blade to make a small pile of super-fine dust, apply with a small soft brush. Easy to grade/fade away, kinda easy to remove (with water/damp brush) if no good, clear-cote to fix. The problem might be a too-smooth surface & a gentle sand with very fine sandpaper not desirable or feasible, it needs a “tooth” ideally microscopic 
Put anything over white and you will change the tone. Not so noticeable 1:1, but stands out on a scale model.
Avoid a traditional all over ‘wash’ and do a pin wash instead. You need to have the white moistened so a very small amount of your detail colour on a brush tip, where the tip just touches the detail section on the part, will be drawn along into the crevices. Add it to a dry parts and the pin wash will be drawn out from the crevice as well, that is why do it moistened.
Keep a clean brush and clean thinner handy to clean up where the pin wash may have bled. Do it before it cures.
When dry, dry brush your white bits with white to restore the hue of the main colour and the highlights if needed.
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