I have only noticed this issue on my iPad 3, 4 and Mini Retina. Never had this issue on the iPad 1 and 2, or on any iPhones since my 3GS, 4, 4S, 5 and 5s
Halogen lighting the screen is white but a little warmer, fluorescent lighting the screen can look cooler, yet still nice and white. And direct sunlight on the screen gives the screen a slightly pink/red tint.
I hate going to the Apple store for screen issues due to their very bright lighting, makes the worst looking screen stunningly white and uniform, take them out of the store and it can look like crap.
Doesn't bother me at all as my eyes soon adjust to the colour change and I don't notice the tinting. Only really noticeable when going from one type of lighting quickly to the next.
Would be interested to know why this occurs.