As it already was said, you are off to a great start.
Good work on with what you did so far

The colors and contrast range you chose with the reference are totally fine.
But you jumped in polishing steps by far to early (e.g. dither)
Where you could improve your artwork significantly is first with the silhouette.
Try to capture the angular sharp look a rock has with a single color.
If you add more colors, always keep in mind which kind of shapes those clusters form. Rocks are mostly quite angular looking,therefore you go with straight lines and sharper angles, not with round forms.
You can add as many colors as you want.
Once you think the result is fine enough you can start with adding dither and AA, but generally those pixel specific techniques come in last, so try to get the big image right at first.
