Jim, I use different mixes of ballast color and "pinch in" the grime, rust and such using ballast. It is tricky to say the least.
I've tried using paint. One time the friggin' dog sat on the air line and I "over grimed", got stuck in one place to long. Painted a bunch of structures when I turned to see wha' happened. Would have made a funny cartoon. Of course I realize that I did things in the wrong order but the weatthering was an afterthought.
Tried using aerosol cans, that got expensive and I left paint in the cans and the overspray was a problem.
I never tried chalks(yet) for the road bed. I use chalks on the engines and rolling stock. If I don't like how it looks I just wash it off and start over.
I learned the method of "pinching in ballast" from an oldtimer back in the late '70's. Works for me, I guess. Just a pinch between the fingers and "pinch it in". Slow but effective. I'm only working on 4-6 feet of track at a time when working with the modules anyway, so it really isn't that tedious, and I don't use much. The Yard was an experience though. But using darker ballast for the initial track bed made that job much less demanding.
I can see if one had a static layout that painting would be much faster. But, again, working on shorter modules makes the "pinching method" more to my liking and i have better control of the amount of "grime" I add.
Bob
PS: I was joking about "dirty" track and using air to clean it. Sometimes people take all of this TOO seriously. lol