War Of The Worlds Diorama - changing colours

So here's the base in Red Oxide Primer and the tanks in the suggested Olive Drab. Had a look at the battle scene in the film (oh, what a hardship) to get an idea what shade of brown the ground should be - and i've got to change ideas.
The ground isn't anything like brown, more just shades of grey, and that's before the Martians start burning things up. So will have to change that.
And the tanks are much more of a lighter green than the Olive Drab, so that's got to change.
I had initially thought any sort of grass would be out as the seperate shards of the stuff i've got would be waaaay out of scale to this 1/44th. But in the film there's quite tall grass there, so thinking i'll put it on after all.