You know, I’m not sure whether it has to do with sort of dithering inherent in 4:1:1 compression or what, but sometimes some small portion of green in certain edges just won’t go away, even after running a full range of channel desaturations. Then if it’s mission critical you’ve got to generate an edge matte and run that through the paces.
I’ve just had better luck with DVCPRO footage. I think that in 4:2:2 color space the values are more purely isolated in their respective channels.
But really a lot of it also happens to do with the quality of the lighting, the lens, on-set spill reductions with gels, and subject to green-screen distance.
You might also want to try a levels adjustment to the green channel.