Owners of the Samsung Galaxy S20 Ultra have complained about a bug that makes their displays assume a green tint when brightness is reduced. It also appears to be present on the regular S20 and S20+. Thankfully, though, Samsung has promised to fix it.
https://www.notebookcheck.net/Samsung-Galaxy-S20-Ultra-Samsung-promises-to-fix-the-green-tint-display-issue.461930.0.html