Hans, this thread is in Developers section so I think rahul wants to do that programmatically.
Even I didn't realize that in my first answer, it should have helped you, rahul, to find out how to do that, using Eclipse Search > File... menu item to search where "Ground color" label comes from and how it's used in the program. Setting the ground color is done in com.eteks.sweethome3d.model.HomeEnvironment class, and to change the default color, you'll just have to call its setGroundColor method when a com.eteks.sweethome3d.model.Home instance is created.