Objects further away tend to be smaller in perspective. The bushes in the first image seem to be intended to scroll at different rated, yet are all the same size, which make the darker bushes look bigger.

The mountain sprites look pretty crude, even for 2 colours.
Could be how you draw the lines on the closer layer.

Not bad! On the third pic maybe add a second shaded color on the side of the blue mountains to give it depth.

Overall you did well. :)