How to make vertex colored squares show black borders

I have a square with four vertices, color of each vertex is different.
in order to distinguish this square from other squares, I want the border to be black color.
Approximately as shown below, the triangle used vertex shader by vtkPolyDataMapper::SetScalarModeToUsePointData(), I also need a black border to distinguish it from others.
how can I achieve this with only one actor.

See vtkProperty::SetEdgeVisibility and vtkProperty::SetLineWidth / vtkProperty::SetEdgeColor.

In order for the polygon to be displayed normally, I need use vtkTriangleFilter to guarantee this.
So if i use vtkProperty::SetEdgexxxx, it will result in the following.
A square only have four edges, but the square shows five sides in the window.
Anyway, thanks for your reply.

You may want to have a look at the vtkFeatureEdges class to get rid of the inner edges.

thanks for your reply.
It will help when it is a polygon, but most application scenarios i used are cubes or blocks. All of the edges are used by two polygon, vtkFeatureEdges can not find out the true boundary.