I have been working on this for a while trying different color variations using a palette I of mine (since I almost always use pre-made palettes), and I was not really sure which one was the most flexible and best looking, so I decided to upload them all...

Note that some grass/rock variations have the same base color, meaning they can (almost) seamlessly be tiled together (using grass as top tile and rock as 'base').

All tiles are 16x16 sized.

...and yes, I was lazy and just mirrored right and left sides :)

EDIT: added bridges and some transitions (third preview and file 'tileset_update.png')

Enjoy!