Wonderful ref. Also helpful for pointing problems.
Adding to ndchristie's point about perspective, look closely how the water edges get more linear and horizontal in the distance. That's a powerful way to convey perspective and depth, linked to the observer's point of view height (not much above water level). In this respect, the further bank in your pixel looks tilted.
About color use: you are using colors you think stuff has, not the actual colors which depend on light, point of view etc. In short, your trees are green, trunks and rocks are brown, water is blue. Not so in life, not so in the ref! There's actually very little green in those trees and grass, very little blue in that water.
But some colors (dark greyish/bluish purples) are reused all over: color conservation is not pixel-art-specific! This is a very strong way to unify a piece.
Distant mountains are more blue than the middle-ground cliff.
Then, shading. Light mostly comes from a low sun behind the trees on the right; it is both soft (no sharp cast shadows) and contrasted (foreground doesn't get much light).
I'd suggest to straighten this before any other consideration, because the dark and light patches have a very strong influence on the overall composition.
Another composition point: look how every part of the painting draw the viewer's eye toward the central details (mountains, cliff castle and waterfalls). The tree on the left is important for this, whereas in your pixel nothing prevents the eye to wander off the image on that side.
