What should I use for a wash on a green ww2/nato vechile?
Wash color? Cover entire model? Slop it on or light coat?
Please all help appreciated, tryin to learn weathering

The wash color, in my opinion, should be based on what you're trying to achieve. Pick a goal first, and then use the techniques to accomplish said goal.

Since it sounds like you're just starting, how about posting a picture (or link) to a model that you like the weathering on, and then we can probably break down how the builder ended up there.

Check out Vallejo Acrylic washes , I started using them a few mths ago. They have different color combinations & I like them alot. They have awash for dark yellow , german grey , vehicles painted green , winter wash etc...
I also like MIG enamel washes but find them too dark & need thinning. The Vallejo Acrylic washes seem just right.
I would suggest experimenting covering the entire model with a wash as well as just small area's etc... there's no set rule .