Night mode is an automatic setting which takes advantage of the new wide-angle camera that's in the iPhone 11 and 11 Pro models. It's equipped with a larger sensor that is able to let in more light, allowing for brighter photos when the light is low.
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Some iPhone 4s Experiencing Display Discoloration?
Image from "elitemrp" in Apple discussion forums
A number of readers in our forum have discovered that their iPhone 4s arriving today are exhibiting areas of brown or yellow discoloration on the highly-touted Retina display. Users in Apple's own discussion forums are also beginning to notice the problem.
Image from "BlakeRx" in MacRumors forums
At least one user paid a visit to the Genius Bar at his local Apple retail store but was told that there are no replacement units available at the moment. Given the iPhone 4 shortages, it may be some time until affected users can obtain replacements.
The prevalence of the issue is unknown at this time, but there are a significant number of users reporting experiencing it on their phones.