Ok so heres an example, shot with my iPhone 8.
On the left is the iPhone 12 pro at 100% brightness.
On the right is my iPad 12.9 pro 2018 at 100% brightness.
Both devices have their true tone turned off.
In real life the iPhone doesnt look pink, and the iPad doesnt look green, this is just how the iPhone 8 sees the image as it tries to find a white balance that mediates between them. It doesnt get the white balance right, hence these somewhat "off" colors.
IF the screen were the same, however, there would not have any difference in color between the two devices. They would both look a bit pink, or a bit green, determined by what white balance was shown.
The iPad 12.9 has a perfect screen, in my opinion. In fact its a replacement unit that I got becasue the previos 12.9 that I had showed an uneven tint.
When holding my iPad 12.9 near my calibrated computer display, I see little difference.
When holding my iPhone 12 pro near my calibrated computer display it is blatantly yellow.
It remains to see if this is due to:
1. A "yellowgate" - as mentioned in this thread it would be Apple sending out units that are way outside a predetermined tolerance / deviation from a target white balance - due to Apple wanting to please customers and get their deliveries on time, even though this means that they are shipping units that they know havent passed testing
2. The white balance on the 12 pro OLED is deliberatley set way towards yellow, for some reason.
I find the latter a bit harder to believe, as it is a "pro" device, markeded towards "pros" and photographers who have needs for correct colors due to the nature of their work.
Also, the iPad is perfectly calibrated, as this is also a pro device that is even more markedet towards graphic design/photo editing/color sensitive work.