[vtkusers] DO I have to use vtkRenderWindow::Render() when I want to change an Actor's color?

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Hi,
     I use vtkUnstructuredGrid to Show lines.when I pick a cell, I want to change the cell's color.I know how to change it, but I don't know how to ask the render to update its new color.I used the vtkUnstructuredGrid 's modified().But it does nothing .I have to use vtkRenderWindow::Render() to render whole scene.it is really slow.
 How to update only the changed actor?
 Thanks
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