Oh my god! My bad
....
I'm so very sorry madartiststan; let me try to undo my damage...
You are on the right track with the low color count, but they should maybe be a bit more dynamic--in other words, each color should hold more weight. Your middle green is good, but you could change out one of the
slightly darker greens for a more dramatically darker green. Since grass is soft, you don't want the highlight to be too bright, you are doing good there.
Depending on how deep or shallow you want the picture to look, it's all going to depend on how you separate each hill. If you want a very vast looking landscape, like what philipptr's edit is going for, you are going to want each hill to be separated with strong dark greens and highlights; but if you are going for a more immediate look to the hills, you are on the right track by keeping each hill close together.
Regardless to the depth you are trying to get though, I would suggest a more gradient sky, because the one bright blue is causing a more "overcast" effect than sky...if nothing else, maybe make the sky a bit more dark and rich.
As for the sun; I wouldn't remove it, as philiptr did, but rather, I'd move it to be behind horizon, half showing--I've never realistically saw a sun in the corner of the picture, but I have seen them on the horizon.
As the piece stands, it's faulty in a few places, but hopefully this and what others said can guide you to make it a bit more solid. Either way, keep pixelling!
