You can get good results without having to go overboard by picking a contrasting tertiary colour and just running the brush sideways irregularly across corners, edges and prominences. So for (say) green tanks, you pick a neutral grey. Yellow models look good with a chocolate brown.
The main trick is not to overdo it — every so often, stop, put the model down and leave it a day before thinking about doing any more.
There’s an alternate approach which is to start with the rust; do a coat of dark brown, tin, orange inks, orange washes, very light silver drybrush and then do the main colour; when you get to the rust section, you “dab” the paint over the border to get a mottled transition.
It depends if your model is to be mostly-painted or mostly-rusted…
Buy a bunch of cheap plastic toy tanks. I found some packs on Amazon that were six quid and each included 4 tanks (and the hundreds of figures and other bits and bobs went to my nephew along with a couple of the commanders painted up in historical uniforms and put in little presentation boxes.)
These are great for practicing airbrushing, trying out cammo patterns and wash/rust/chipping effects and so on.
 Cos I am an awesome aunt. He’s 4. He doesn’t appreciate this.