Even though this post is pretty old at this point, I thought I would throw my 2 cents in. I don't know if you have resolved your issue yet, but I also have the VZ GNex and recently saw the exact same thing happen. I do believe this is a software problem as I flashed a custom ROM to my device and the problem was solved. For how long I do not know, I just did this moments ago. I was running a custom Cyanogenmod built on 4/22, v4.0.4. When I saw the purple tinted business on my display, I played around a bit to see if I could correct the issue. I played with the brightness, saw the same thing you saw. I did notice though that when I opened the app drawer, the background there was completely black - no sign of the purple at all. This made me think it had something to do with the software. So I restored a previous backup and sure enough my display looks good again and the brightness worked as it always did. 

I will reply to this chain if anything changes.