Here are a couple of quick mock-ups I put together and photographed just now using Gaugemaster (Noch) Beige Rough Grass and Woodland Scenics foliage. The foliage in the first photo is dark green, and in the second photo it's medium green.
The point of these photos is purely to compare the colours. What I'm looking to do is find the best combination of grass and foliage colours to represent late summer.
The medium green foliage seems to look better than the dark green - at least against this grass colour. Having said that, the grass might be almost too yellow - it could almost be a field of wheat. Ideally, there needs to be a little green showing through beneath the longer yellow-brown grass, but using the Guagemaster/Noch product means you're limiting yourself to an all-over single colour. Still, its not bad at all.