Bad News for you.
I (and some of my friends too) have the same Bug.
I deleted the Map files and the Cache files but it was still there.
After that I reinstalled the game... but grass is still black.
here you can read about my thread: http://www.gamesas.com/forums/viewtopic.php?f=23&t=16911
At the moment it seems that there is no fix for this.
