Get it right in full light first, sort out the lighting later. Your surface layer for the grass seems to be two hard colours of green and brown, however you've mixed them, there doesn't seem to be shades, rather just shadows. I'd be tempted to try and get patches of difference in the greens. Something I picked up on these forums, which I've got to say is quite brilliant, and I forget who suggested it, sorry, is to get a reference image of the terrain you're basing your scene on, then convert it to a gif, with a very limited colour pallete, something like twenty shades. A bit like posterising, so that you can colour pick key colours in the spectrum of your reference image.