Im assuming that its an Apple LCD
If not what make is your LCD?
How old is it?
It sounds like dust and oxide are creeping in, perhaps from improper sealing of any of the multiple lamintes that comprise an LCD screen.
If it has a cold cathode tube I cant see the reason for only the outer edges to yellow. If the light pipe is oxygen sensitive, it might account for it yellowing around the edges.
EL backing also ages uniformly, but most LCDs dont use this technology.
If you or anybody else has ever taken the screen apart, you might have improperly put it back together. Perhaps the screen is pressing to tightly against the frame. This would cause tweening of the matrix and bending of the light pipe, and ether one could account for a localized spectral shift.
I am just throwing out ideas to see what people think.