Hi,
The first thing to remember when using the grass bump color is that it is always going the make your terrain drop down to the Z of the grass bump color you have selected to use. So really think of the grass bump colors as more like "holes" or "craters".
If you take an area of your map where your grass is at a Z level of 10. And make a decent sized circle using Grass bump Z 0 -- this will cause the area on your map where the Grass bump color was applied to drop down from a Z of 10 to 0 and will have cliff edging to cover the drop. Thus creating a cliff between the grass color and the lower grass bump color.
In order for the grass bumps to work properly you must always use a lower Z grass bump than the adjacent grass tiles.
If you wanted to create a section within the circle (grass bump) that raises back up then you would just use a Grass color with a higher Z than the Grass Bump color used.
I hope this helps you out. They can be tricky, but once I started treating the Grass Bump colors as "holes" or "craters" cause that's really what they create I found developing them was much easier.
Carl Hamilton (Imprint)
http://members.cox.net/darkshard/