I have had the same problem for about 6 months, and never paid much attention to it. I also noticed that when trying to change the menu color, the colors do not show up correctly.
Now it seems to be getting worse, and so I started doing some research, starting with the Sony site. That is when I found out there is an extended warranty for the KDS series (50a2000, 55a2000, 60a2000, r60xbr2, and r70xbr2) TVs because of an optical block issue, until June 30, 2010, no matter when you bought the TV.
You can see the recall here: http://esupport.sony.com/US/perl/news-item.pl?news_id=277&mdl=KDS60A2000
It seems they are extending the warranty, but not doing a recall. In other words, they aren't telling anyone, but if you happen to find the information, they will replace the optical block, if necessary. The closest I could find to the green hue issue is a purple haze on the screen.
I don't know if this is the problem, but I have scheduled an appointment to have the optical block changed. I will repost after the optical block has been changed and inform whether this is the fix to the problem.
Either way, the optical block seems to be an issue and would get it replaced whether you need it or not.