Hi,
This sounds like the true tone display is on your device except I am not sure about functionality roll out on iPhone 7.
My best suggestion would be go to: Settings>Display and brightness>True Tone> Switch to Off.
If this does not solve the problem you could have night shift switched on unintentionally Go: Settings>Display and brightness>Night shift> See if it Scheduled and switch off or switch off manually enabled night shift. You can alter a colour temperature bar to see if this solves the problem.
Once you have done both of these I would recommend restarting your device by holding the lock button and the volume down button simultaneously until the apple logo appears.
Hope this helps.
Thanks
Alex