Clear coats & oil paints - adding color variation to my car body.

Actually I have an idea I want to try tonight.

I'll leave some black acrylic paint lying out and let it dry up, then use a soft brush and apply it very gently to the model. I'll have to practice on a piece of paper first to make sure I'm not gonna put ugly black streaks on the thing...

Maybe try airbrushing some Tamiya X-19 Smoke over the body, and then put on the satin clear coat?  You can build the X-19 up in layers a little bit at a time to achieve the desired effect.  I'll be using that on my F-16 radome to give it the look of rubberized radome paint that has some grime on it from being in service.

Not sure that I'd go either of those routes.   I think that either would be a sure fire means of mucking it up.  If you choose to proceed, think about applying a barrier between the underlying paint and the wash.  Future floor polish is the common recommendation   

I think my approach would be to use a method similar to that used by aircraft modelers doing natural metal finishes. Mask and spray selected panels with an alternate color.   The color should be similar to, but just different enough, the base color.   You may have a problem if you used a rattle can, but if you used a separate paint you can add a drop or two of dark metallic paint to the base stainless to adjust the color

@EdGrune

Yeah, I think I may be on the verge of a major disaster if I don't get this right.

I tried the dip-in-dry-paint-and-apply-lightly technique on a piece of paper. You're right, it either didn't have an effect or, when I pressed harder, started to come out splotchy. So that's out.

Eaglecash867 is right in that I should just airbrush it on, very very lightly. The problem is when I do airbrush on a low setting, the paint alternates between coming out finely or spurting out. So that's also out of the picture. 

I'm not sure why my airbrush does that (too cheap?), or if it's truly the low-pressure that's causing it. When I watch those Sideshow Collectible videos on YouTube, they add very fine shadows to the figurines in that way (with the airbrush pressure set to verrrry low). Maybe it's coz my airbrush is a siphon airbrush that I can't do the same? I'm definitely not understanding something here...

Someday I'll have to figure it out. For now, though, I think I'm just gonna cash in my chips and complete the model without the effect. Live to fight another day. Or whatever other metaphor you can think of. This is the closest I've come to completing a model for quite a while, and I do think I'm on the brink of something display-worthy (which would make it only my second display-worthy model), so I'm being very careful how I proceed with what's left. 

You can do shading with a siphon-feed airbrush too.  You just have to get your paint thinned down enough so you can shoot it at lower pressure.  Probably a good plan to quit while you're ahead though.  If you haven't already done so, get a box of plastic spoons and put some primer on a bunch of them.  After that, you can practice different techniques and effects on the spoons, without trashing the work you've already put so much time into on your models.