I like to use pastels. It's not a perfect system, but I'll take a pastel and rub it on some cardboard (like the inside of the model box) and then I'll take a q-tip and rub that on the resulting mess, and apply it gently to the plane, such as black soot color around the engine exhaust stacks.
Sometimes also white or light gray depending on the color of the plane. To match various photos. And sometimes some brown in there too.
I use reddish brown on wheels to simulate dirt / dust. Light flourescent green to help create the impression of fuel corrosion like on Corsairs based on the Solomons.
I'll usually spray paint models aluminum color prior to painting with acrylics, then some acrylic paints just chip off from handling, more comes off with a bit of brushing in key spots with a tooth brush. Looks more natural to me than painting the silver tidbits on.
Some more pics with some soot etc.