S21fe display green line

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In low brightness situations, dark areas of the screen may appear discolored or stained, or to have a slight green tint. This is normal and does not affect the performance of the device.

The AMOLED screens in most Galaxy devices are capable of displaying a wider range of colors than a backlit LCD screen. If the screen is filled with black or very dark colors, the brightness setting is very low, or the device is in a dark environment, small variations in color may appear as discolored stains or green areas on the screen.

This is not a defect of the device and device performance is not affected. Photos of the issue generally appear worse than what actually appears on the screen itself because cameras typically enhance the contrast in low light situations.

There is another related problem that is affecting customers; sometimes the green tint will appear while they are browsing web pages on Samsung Internet or Google Chrome. The tint may appear while playing certain content, including GIF images. Samsung is aware of this issue and is scheduled to release new software to address the issue in the near future.

In the meantime, customers can eliminate the green tint by increasing the brightness or setting the Motion smoothness to Standard.

Customers can also disable Dark mode and use Light mode instead by opening Settings, then tapping Display, and then tapping Light

Note
Continue to check for software updates to improve the device's performance.

If that doesn't help, this link might help better.