According to a batch of complaints discovered by Macrumors, several iPhone users have been complaining of a greenish tint on their displays that they've pinned on either a recent iOS update, or a defect in the phones' hardware, the latter of which requires a new display.
Here's a good illustration of what we're talking about. If you think you might be having the same issue it's also helpful for comparison's sake:
The models most affected are the iPhone 11 Pro and the Pro Max, but some people stretching as far back as the iPhone X (which came out in 2017) have had the same problem. Forbes adds that the thread that ties these models together is their OLED displays.
Some people who have experienced the issue say that it first came around when their phones got the iOS 13.4 software update. It was made worse when the 13.5 and 13.5.1 updates took effect. Others have said that their phones have been messed up since the day they were purchased.
While this would seem like an easy fix for the eggheads who work for Apple, the appearance that this can be both a software or hardware problem complicates matters. For example, screenshots taken on an affected phone don't show the greenish tint when viewed on a different display. That suggests it isn't a software issue. Relative to that, in some instances, Apple is said to have approved their official resellers to replace displays covered by a warranty.
Forbes reports that the problem is still present in Apple's new iOS 13.5.5 beta, while Engadget relayed the experience of a Reddit user that said it went away after they installed that update. Again, that could speak to the problem's relative complexity.
At the moment it doesn't seem like there's any timeframe to wait on for a fix, but if Apple relays their intent to do just that then we'll update.