On the terrain the brush does not appear, no chance to painting on terrain. But on one of the mesh-planes in your example project, where the 4 texture shader is attached, the removing, adding grass works well. I guess, I would use that mesh based shader anyway How do you use this asset? On copy of the map (with just 4 grasses?), or on a mesh, which can be dropped (mesh deformed based by the terrain) frequently with different grasses on the terrain?