I have gone through 6 ipads and all of them had some sort of issue with the screen. Two of them had dead pixels and 4 of them had uneven screen temperature (like the OP). In one of the apple stores they only let me swap 3 times and twice in another. Got the 6th one from staples and it has an even (yellowish white but it is same as real paper color!) brightness but I just spotted a dead pixel. I am gonna swap for another tomorrow and hope for a good screen. I swear this is my last try
BTW units starting with dmj have LG displays(yellowish white) and dmq have Samsung(blueish white) displays.
My Ipad 4 has a warm screen temp but isn't as uniform as my ipad 3,slightly brighter on the left hand side when in portrait,infact this is the first time I've had a really noticeable non uniformity in the screen on an IOS device for quit a while (ipad2,iphone 4 & 4S,ipad 3 all were good)
My experience of IOS device displays has been :-
Iphone 4 cool - 4S warmer - iphone 5 warmer still
Ipad 2 cool - 3 warmer - 4 warmer still