This is an odd one, I find that randomly things shade over or go grey if I hover over certain things but it's mostly with Flans mod items in my inventory either creative or survival and in the NEI GUI as well.
here's 2 pics showing me hover over the flans mod section in NEI and not hovering over it, I have not tried any older versions as this is part of my modpack and all smp parts packs are in there.
The first pic shows the background behind the GUI has changed to a weird dirt and sky area but the second pic shows the proper view where I would be looking at when the GUI is closed.
It's a bug with NEI or OpenGL.
Sometimes when I hover over it everything goes white except for the GUI for a second, after the screen returns to normal it will say OpenGL stack Overflow 1283 in the chat, probably something with the models.