"Green tint" is an issue in which OLED displays seem to glow where they should remain completely dark under certain conditions. It was present in some units of the iPhone 11 series last year, and has now reportedly raised its head again in the new 12s. It may affect any of the flagship line's 4 variants, and has yet to see an update that might fix it.
https://www.notebookcheck.net/The-iPhone-12-series-may-be-the-latest-premium-device-line-to-exhibit-the-green-tint-display-disorder.505387.0.html