iPhone 6s display color profile differences between devices
Do you know the history of each of these iPhones?

This may not necessarily explain the difference in your case, but one factor that can result in this difference is the use of non-genuine parts. For example, I've seen this type of differentiation in iPhones that have had screens replaced with 3rd party screens.

Otherwise you could check that the following settings found in Settings > General > Accessibility are the same in all iPhones:


Display Accommodations > Colour Filters
Display Accommodations > Auto-Brightness
Display Accommodations > Reduce White Point
Increase Contrast > Reduce Transparency
Increase Contrast > Darken Colours