the greens - I marked them on your picture. It´s the light green on the ground. In the background it works I think, but the same color in the foreground looks strange to me. I am not sure - probably its because usually when looking on grass like stuff from above, it gets darker - because you see more from the ground. From lower height with flater view angle you don´t see so much from the ground and so it looks greener. So if the background stuff is light green, then the same material/plants/grass/whatever it is should look darker in the front, because you see more from the ground.
By the way, something similar can happen with a color shift from green to brown - when you have blossoming grass. In that case, when looking from above/steep angle, then it looks mostly green. But when looking in a flat angle you will see more of the blossoms which often are brown - so in that case the distant color would be more brownish.
And of course you also can have a mixture of both effects ;-)
Definition with trees: if you look at your reference picture, you will notice a higher range from bright green to nearly black green in the tree crown. Towards the center of the trees the crown almost gets black. The outer areas tend to be much brighter, especially when sunlight comes from the back, they almost have some sort of halo.
The trees in your rendering are to even, not so much brightness contrast in the crown. And I think there probably could be some varying hues also.
Please let me know if I should send you the marked picture.
As I said, you already did a great job and i am sure that your customer is really happy!
