My iPhone 6S display is warmer than your other Apple devices, and after comparing it to other 6S demo models in store it's fair to say that's just how displays are for that particular model. If you've got a 6S and another iOS/OS X device, open up the calendar app on both devices and have a look at the difference in temperatures, it's unusually un-Apple how this display has somehow made it into production.
However it's the only device in the line I'd say definitely has a warm display temperature, when comparing it to the new iPad Pros and the 6S+, they still have the cooler, blue-ish screen temperature. True tone is a life saver though, I'd definitely prefer having a bluer screen by default then having the option of true tone and night shift to warm things up manually.