iPad screen goes from white to yellowish

Hi all.

I've had this iPad since the summer with no problems to speak of. However, I've been reading more books on it and tonight noticed that the screen (divided lengthwise) is bluer and whiter on one side and yellower and browner on the other. This difference is independent of how the iPad is rotated, i.e., the yellowish-brown half is on the side where the volume button resides.

It's not interfering with my ability to read, but it is very obvious once you've noticed it. Is this a flaw or a feature? Anyone else have this problem?

Thanks.

P.S. I saw another thread with this issue dating from May, but there was no further comment. Also, this is a well-used iPad, not a new one, so any of the initial discoloration issues shouldn't (I don't imagine) apply.

It's always present, but it's by far most obvious when the background is lighter such as when you're reading books and mail. It's not a sharp division, more a gradual move from a bluish, colder hue to a yellowish, warmer one. If that makes sense.