Well also the way Chameleons work ( the lizard) you would take on the coloring & texture of the grass, so you would be visible as a large humanoid shaped lump of grass (unless you crouch down of course), really, think about taking a Chameleon and tossing it in the air, or levitating it. Would it take on the aspect of the sky? not likely. So unless your against a surface taller than you, I would think you would maintain your shape, just the texture and color would alter kinda like a 3D character. So it wouldn't really be ideal in an open field (unless crouched down again). At least thats how I always treated it in games