// ...
def setGreenBkg(mtl) {
  mtl.setDiffuseX(0);
  mtl.setDiffuseY(1);
  mtl.setDiffuseZ(0);
  mtl.setAmbientX(0);
  mtl.setAmbientY(1);
  mtl.setAmbientZ(0);
}

def setRedBkg(mtl) {
  mtl.setDiffuseX(1);
  mtl.setDiffuseY(0);
  mtl.setDiffuseZ(0);
  mtl.setAmbientX(1);
  mtl.setAmbientY(0);
  mtl.setAmbientZ(0);
}

def setCyanBkg(mtl) {
  mtl.setDiffuseX(0);
  mtl.setDiffuseY(1);
  mtl.setDiffuseZ(1);
  mtl.setAmbientX(0);
  mtl.setAmbientY(1);
  mtl.setAmbientZ(1);
}
