tube.CappingOn();
vtkPolyDataMapper mapper = new vtkPolyDataMapper();
mapper.SetInput(tube.GetOutput());
mapper.ScalarVisibilityOn();
mapper.SetScalarModeToUsePointFieldData();
mapper.SelectColorArray("colors");
vtkActor actor = new vtkActor();
actor.SetMapper(mapper);
actor.GetProperty().SetOpacity(opacity);