I've got two citadel (games workshop) grass battlemats that I sit on the table, then sit scenery on top of them. However, compared to flocked and painted tables, it is rather plain. I decided to convert one of the mats into european/british fields by lightly brushing the fake-grass mat with different browns and greens. I wasn't sure how it would turn out, or if it would even work, but it seems to have worked!
I started off by zooming on on southern england in Google Earth to see what farmy fields might look like from above. Then I lightly painted lines of dark angel green to form the outline of the fields.
Then I brushed lines of dark angel green onto the surface of the fake grass mat.
Then I tried some other colours, such as desert yellow and camo green.
Then I did bestial brown to finish off, also dabbing it over the outlines of the fields.
Then I can lay out scenery on top of it, and it no longer looks like a big flat green expanse!