iPad Air 1 / iPad Air 2 screen color differences

Hi

I have two iPad Air 1's, fairly new, and just got iPad Air 2. When kept side by side (see pictures below: iPad Air 1 on left, Air 2 in middle and another iPad Air 1 on right. See the color differences on screen? How do I know which one is accurate? Why do the screen colors vary for a giant like Apple which is in pursuit of perfection?

Note that the Air 2 has now been returned: After placing the screen protector, the "superiority" of Air 2 screen was diminished and I found lesser difference between Air 1 and 2 screen, not worth spending the extra $!!!

See pictures below. Please also visit http://displaymate.com/iPad6_ShootOut.htm that says Air 1 usually has 7,041K vs iPad Air 2 that has 7,355K color temperature of screen, so iPad Air 2 (in center below) should have been more "blue-er" than the Air 1's on either side which does not seem to be the case! I am posting this to ask experts how to determine which is most accurate screen color and how to make sure I get iPad from the "right" batch? Any help/opinions? Thanks so much!