Ambient color not working in VTK7.1.0, while fine in VTK6.3.0

The ambient color composition seems broken in VTK7.1.0, but OK in VTK6.3.0. The attached python script can demonstrate that.

An actor that has non-zero ambient, zero diffuse and zero specular is created. There's no light, but the renderer's ambient is set to green.

The result should be green, since global green ambient shine on white actor.

While in VTK6.3.0, it shows green, in VTK7.1.0, it shows grey.

I'm using Win7 x64, both versions of vtkpython are downloaded from the official release page.

Run this: vtkpython.exe test_light.py

test_light.py