Mud
Pre-shading is pretty much just that - prime the model, spray flat black onto the panel lines and recessed detail. When you paint the color coat, thin the paint a bit more than usual and mist it over the model. You'll fins that the preshaded areas will be a little darker than the middle of the panels.
Rust is harder. Some use thickened paint - red/brown with a black wash can look good. Some people use a bit of talcum powder in the paint to give the area the lumpy/flaky look of rust.
Off colored panels are just that - mask off a panel and paint it a slightly lightened or darkened shade of the original base color.
Pastels are great to use. Make sure you get the dry type - not the oil based type, cuz you'll need to rub the pastel onto rough wood sandpaper to create little piles of dust. Black, brown, red, gray, green and white are very helpful to blend into about any color scheme you need. You need to apply the pastel dust to FLAT paint (it'll just fall off gloss paint) and kinda brush/rub the pastel into the paint. Black, brown and gray are great for exhaust on WWII aircraft.
Kalmbach have a number of books that have heaps of different types of techniques for adding realism. Also, comsider subscribing to FSM - it really is a good mag - very instructional. I've been subscribing since 1982 and haven't regretted it for a moment.
Just experiment a bit and have fun. Don't try to make every model a contest winner - that takes the fun out of it and only frustrates you in the end. Like I said, experiment and have fun.
Cheers,
Lee Tree