Using 8bit colors in prompt expansion on a terminal with 24bit color support

[Thayne <astrothayne@gmail.com>]

I recently tried out powerlevel10k, but ran into something weird. The
classic theme had blue backgrounds, where it was supposed to have gray
backgrounds. After some investigation I discovered it was because the
`%K` expansion just passes the value through to the setab terminfo
format, which in the case of "direct" terminfo entries
(alacritty-direct in my case, but I think it would be the same with
xterm-direct) assumes it is an RGB value if it is greater than 8.
Apparently this is intentional behaviour
(https://lists.gnu.org/archive/html/bug-ncurses/2019-03/msg00009.html)
for terminfo.

Is it expected that zsh's prompt expansion for colors behaves this
way?If so, maybe the documentation on it should be more clear that it
is the users responsibility to make sure that you use 24bit colors
(with hex) rather than 8bit colors if the terminal expects it (by
checking for the RGB property in terminfo). Or should zsh map the 8bit
color to the corresponding 24bit color?