Ah I see. The sand terrain set does not have a grass texture.
You could create your own, combining the best of the terrains textures that fit your needs.
Duplicate one of the terrainbank folders and rename it.
Rename all assets to the same name.
Copy in the textures you want to replace.
Erase to old and rename the new to the same name as you are replacing.
Erase the superpallet.ter file.
Open GameGuru and set a start marker.
Run test game and then go into the matrics panel (tab tab)
Choose the new terrain from the terrain dropdown list.
Now you should be able to paint things as you want.
The default texture is the grass texture in the default terrain.
You will need alpha channel for all _D textures compressed at dxt5.
Hope that helps.
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