If you're trying to replicate worn paint where the metal underneath is showing through, use some sort of silver paint mixed with a bit of black. You don't necessarily want a screaming bright silver. Tone it down a bit with the black and experiment to see how much to use.
You can apply it straight, that is simply brush it on over small areas. Or you can dry-brush it and get a blended look. This is usually effective for areas where crew or infantry may enter or exit the helo.
For the soot, these guys already mentioned my fave; pastels. I use a variety of chalk pastels (not oil pastels) to replicate a variety of things, including exhaust staining and chordite streaking.
I use a few different brushes to apply the stuff, my favorite being what's called a Cat's Tongue. Filberts are also good, but any brush that you're comfortable with is what you should use. I don't recommend applying pastels with your fingers because of the oils in the skin.
You can either apply the stuff with a streaking motion, or a swirling/scrubbing motion. I just go with what feels good, I've been doing this for so long.
There are many techniques you can try to in order to get the desired results. I'd recommend trying as many as possible to see which may or may not be "for you".
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