How are you performing this test? Are you creating an application with a plain grey window and running it?
I think this could be an OS issue/graphics driver issue. I sometimes notice things "behind" my dark grey Xcode editor window - like for example, I can see this website in Safari behind Xcode. However, sometimes, I see stuff that isn't directly behind Xcode - but it is in another Space.
If the hardware test is succeeding, and you are only seeing "ghosting" on particularly colored windows, I think it is safe to say it may not be the display. (It still may be the display. I'm just saying there are more variables here, and I don't have all of your information.)
I think this could be an OS issue/graphics driver issue. I sometimes notice things "behind" my dark grey Xcode editor window - like for example, I can see this website in Safari behind Xcode. However, sometimes, I see stuff that isn't directly behind Xcode - but it is in another Space.
If the hardware test is succeeding, and you are only seeing "ghosting" on particularly colored windows, I think it is safe to say it may not be the display. (It still may be the display. I'm just saying there are more variables here, and I don't have all of your information.)