[Sjonny]

COLORTERM is also a very weird thing. Most of the time I don't need it to be defined, but when using a screen session it suddenly looks very crappy when COLORTERM is set to truecolor. Removing the env var makes it look all normal again.

[spudlyo]

Tmux supports 24-bit color. I bit the bullet and switched a few years ago, and my muscle memory remains exactly the same. I'm incredibly happy with the power, flexibility, configurability, and reliability of Tmux.

[bhaak]

Does your screen even support 24bit colors?

If unsetting COLORTERM fixes the display problem then it is a program issuing 24bit colors but the terminal (in your case screen) not understanding the escape sequences.