So I got a new iPhone 6s and used to have an 6 before.
So I just put the 6 and 6s side by side to compare the displays and noted that the 6s is yellowish!
Some backlighting LEDs have different color temps. I think Apple tried to keep the same color temp LEDs in each phone but some batches are more warm in color while some are cooler. I prefer slightly cooler or blue color temps.