My iPhone 7 has darker colors

You seem to be suggesting your screen is showing more contrast than some other iPhone which is your reference. This is not a valid comparison, there is no “normal iPhone” because almost every model has a different screen with different resolution and contrast range, even before you consider user settings like brightness level auto dimming with ambient light, and even the ambient light itself which will influence your perception unless you compare side by side.

If you are unhappy with your phone’s screen Contact Apple Support to arrange a Genius Bar appointment and get the phone examined by Apple’s techs who have test equipment and know the devices better than anyone else around.

Ok, see the last paragraph of my previous reply. Get it checked by the experts.