Thanks to all for the replies. The green tint is there at all resolutions...640x480,832x624,1024x768...and changing to black and white makes no difference, nor does editing the desktop background pattern. I will try cleaning all connectors, but unfortunately the video cable is hard wired to the monitor and I only have the one Mac-VGA adaptor.

As I said, readability doesn't suffer, it's just annoying, but since the monitor was cheap (ie, free) I'll not complain too much.