Most iMac and IPS monitor that I saw has the bottom 1/3 yellowish.. Weird
What you've seen may be something about the manufacturer of the specific IPS panels. I have two Asus 24" IPS LCDs for my work PC - both are uniformly white to my eyes - but both are probably from the same manufacturer (LG, maybe).
The IPS technology itself isn't known for having color variations or anything. If anything, it's the opposite. The color reproduction and viewing angles are the selling point of IPS with black levels and response times being their weaknesses.
If there's anything that IPS displays normally suffer from (just as a technology), it's whitish glow in the corners when viewed from an angle. At first, it looks like backlight bleed, but it shifts as you move around and usually disappears entirely when you get directly in front of an inline with the display.