The OLED screen of Samsung Galaxy Note 20 & Note 20 Ultra has a weak point that it is very easy to render the screen color. In the case of the Note 20 and Note 20 Ultra, the most common shade is green.
This error occurs because the screen brightness is reduced to a certain extent, when the background shows a gray tone. Both the Exynos and Snapdragon versions of the Samsung Note 20 phone can experience this bug.
This is a software issue, which can be fixed with an initial update. In the near future, there will be a software that can completely fix this blue error released by Samsung. You can still fix it by keeping the screen brightness high. If there is still blue color at high brightness, bring the phone to a service center for replacement.