As a simple test, I created a 5x5 grid plane, and attached a simple black-to-white gradient texture to it. Then using uu3d, I converted the texture to vertex colors. Then I changed the gradient texture to another texture and saved it as a gmf. After creating the mat file with the sway shader, I placed the model into an editor scene. The edge of the plane that had the black vertex colors did not move, while the opposite edge with the white vertex color made it move the maximum amount dictated by the sway shader. The gradient vertex colors in between varied in movement with the lighter colors moving more than the darker colors. Also, since I had a texture actually assigned in the gmf/mat file, it didn't show a black-to-white gradient but the texture that I had assigned.
Also, I agree that black may be too much in some instances as well as the white would be... but for a grass root, I would assume one wouldn't want that to move at all.