> I think Zsh doesn't support truecolor internally.
Doesn't that: https://i.imgur.com/FDFf8jn.png mean it does?

> The color names or numbers are used in region_highlight array in strings
> like "fg=110,bold" and here there is no way to specify 24 bit color value.

Yup, that's what I talk about: it would be nice to have an ability to specify 
them. Although, I even agree on some shell env file with [magic] definitions 
instead of ini to make it work at least in my dotfiles.

> I was earlier looking for 24 bit color support in ncurses, and it's
> rather the same story, the library is fixed on 256 value integers and
> cannot be easily adapted to support 24 bit values.

Yup, there was some problem about truecolor in ncurses, but since 6.0.20180121 
(or, probably, a bit earlier, since I use that in neovim-under-tmux for a long 
time already) it does support truecolor through terminfo's `Tc` (AFAIRC) 
capability.