free to use
For grass, I offer up 3 techniques that anyone should be able to use successfully.
1. Make a plain green tile. Then make another plain green tile, but on this tile draw a small tuft of grass. Then fill the area you wish to be grass with the green tile and go around scattering the tuft tile in the plain green.
2. A simple checker pattern of 2 different greens can make for a fairly nice looking grass.
3. The simple airbrush method. First, start with a plain green tile. Then take another shade of green and air brush lightly all over the tile. Then do this again with another shade of green, preferably one that is fairly close tot he other 2.
Next, select the pencil tool and go in and break up the large clumps by painting in with the original green color so that only 1 pixel of each of the secondary airbrushed green colors are touching.
In order break up the tiled look, don't waste time making more than 1 tile. Simply take the tile you already made, copy it, then turn the copy 90 degrees. Do this for each possible direction and suddenly you have a select of 4 possible grass tiles.
Also, on a brief and slightly different note. One should avoid neon greens for grass. Background tiles should try to be darker and less bright than the characters, npcs, and objects in a game so the player's focus goes straight to what matters.
Hope this all helps, and as I stated above, feel free to simply use these grass tiles I posted here. The picture is already in OHR tilemap format, so help yourself.