Well, it's a good start, but in my opinion there are some things you can improve.
- First of all, the difference between the darkest purple and the darkest brown is too large, it maybe would be better if you increase the number of colors to make it looks finer.
- The colors are well selected, but with just that colors the rocks don't look "deep", I would add a bit more of a darker color, and use more dark to make profundity with shadows.
- I don't like so much how grass looks actually, sorry...
Here an example (my rocks aren't awesome but
I show them just for the palette)
Yes, the grass is missing in mine.
* Maybe someone will disagree *