Adjusting the colors is only the first step.
The first version is very polluted, with several random and isolated pixels. The latest version is best to work from then on. Now, you need to define a point of light, and match all pixel-art elements to it, using shadows and lights. Work better the mountains, the clouds and the ocean during this step, using one or more references for each. Just do the sketch of these things. Try using a thick pencil / brush to do so. No detail, only a small sketch of what will compose the piece. For example of what I'm saying, take a look on the tree of a kevink's sketch with the Dawnbriger pallete:
As you can see, is unrefined and is NOT a finished work. He just made a sketch of the shadows, textures and lighting that will compose the piece.