iMac 27 5k Late 2015 - display halves have different tone

Hello, dear Apple community!

I have a display problem with brand new iMac 27 5k Late 2015 which I bought 3 weeks ago and already replaced it 2 times. Which means I have already checked 3 similar models and they all have the same problem which can be easily spotted on attached photo: left half of the screen has slightly different color than the right one.

https://www.dropbox.com/s/hdqmi2p377nxm6m/imac.jpg?dl=0

I placed Calendar window on purpose to visually divide the screen - 6 months on left and 6 on the right - it makes it easier to see the problem on photo.

Does anyone else experienced the same?

Apple support has no idea what it is. Moreover they haven't heard about such complaints before. Although I am replacing my device for the third time now.