Thanks for your feedback. I'm quite new to pixel art, and I don't really know how to add details without using too many colors. Do you have any idea how I could do that ?
The amount of work you put into the leaves make the rest seem unfinished and flat.
some dithering in the sky would help smooth out the gradient tone for starters c:
look at the red into green example on this page http://www.furcadia.pixelovely.com/dithertut.html