This thread might be of interest to you:
http://forums.graalonline.com/forums/showthread.php?t=87425
edit: And an edit for safe measure!
I didn't touch the colors and I didn't bother with making it tile-friendly. However, what you want to try to do with working with a tile like this, that has multiple colored stones... is to make different colors, but low contrast. That way while the player looks at it tiled, they don't see a bunch of bright tan, dark blue, neutral gray and so on. Instead what they will see is a blend of slightly different colored stones, that do enough to give variety, but don't fight each other for attention.
You also want to focus a lot on smoothing the cracks with the bricks, so that the lines themselves do not cry for attention. Then you can do a simple, but effective method of spotting dark black pixels in between intersections to easily portray a depth to the stones. As well, you don't want to give the stones depth as very rarely will they actually come out of the ground and it's harder to make it look decent, and easier just to make them flat with a touch of highlights.