There's plenty of control over the color of the terrain through slope and height, but what if I want a certain region along the X & Z axes to be a specific color, or even have its own unique distribution shaders?
best way is to use functions.
for a distribution within x=+100; x=-100 you would use this:
(http://img442.imageshack.us/img442/7010/23091752.jpg)
as a blendshader for whatever you want.
Or you could just use the Simple Shape shader.
or just paint the area with the painted shader.