object on the fly--how to change color of particular vertex

[hzhao]

With the following code, I created an object on-the-fly, a green polygon. Then I want to create a vertical transparent stripe, that is, set vertex 2 and 5 to be transparent. But the code doesn't work this way. Why? Thanks!

viz.startLayer(viz.POLYGON)
viz.vertexColor(viz.GREEN)

viz.vertex(0.0,0.0,0.0)
viz.vertex(0.5,0.0,0.0)
viz.vertex(0.6,0.0,0.0)
viz.vertex(1.0,0.0,0.0)
viz.vertex(1.0,1.0,0.0)
viz.vertex(0.6,1.0,0.0)
viz.vertex(0.5,1.0,0.0)
viz.vertex(0.0,1.0,0.0)
target=viz.endLayer()

target.dynamic()

target.setVertexColor(2,[0,0,1,0.2])
target.setVertexColor(5,[0,0,1,0.2])