GNU Emacs 28.1 has broken colors when opened in a screen session with the COLORTERM environment variable set to COLORTERM=truecolor.
To reproduce:
1. Open a terminal that supports 24-bit color (e.g. GNOME Terminal)
2. Run export COLORTERM=truecolor (this is already set by default in many terminal configs)
3. Create a screen session by running screen foo
4. Launch GNU Emacs 28.1
5. Run M-x list-colors-display
Expected behavior: Emacs shows the full spectrum of colors supported in the current terminal
Actual behavior: Emacs shows the names of the full spectrum of colors, but almost all of them are displayed in various shades of gray.
Workaround:
Add unsetenv COLORTERM to ~/.screenrc
The root cause is that GNU Emacs 28.1 added code that enables 24-bit color if the COLORTERM=truecolor, but screen doesn't actually pass through 24-bit color values correctly.
Until screen has support for 24-bit color values, it would be nice if it automatically unset the COLORTERM environment variable during startup.