osm2world green terrain

Setting the createTerrain parameter to false in your OSM2World configuration file will remove the green default terrain entirely. This and other configuration options are documented on the OSM2World configuration file wiki page.

Slightly OT: Please also remember to download the example textures and config file which are available as a separate download from osm2world.org/download/. This will give you improved visuals compared to the flat colors you get from an unconfigured OSM2World.

The OSM2World wiki mentions a configuration file which you can specify via option --config <file>. Inside the configuration file you can set a backgroundColor or alternatively a backgroundImage.