To enable true color in tmux, add this to the config

# Assuming your terminal emulators outputs $TERM as "xterm-256color"
set -ga terminal-overrides ",xterm-256color:Tc"

• Github issue on true color support. After much work and discussion, this comment, which is the above example, would do the trick. I haven't identified the strings of PRs that implemented it yet.
• End 2 end explanation of color support from terminal, terminfo, tmux, and vim.
• True Color test and support

Assuming your terminal emulators doesn't have true color, this may become a problem. Certain emulators doesn't sit well with tmux and tmux would fail to display bold or italic text properly. This can be fixed by monkey patching the $TERM variable that tmux uses and create a terminfo for that specific $TERM.

Add the terminfo entry to the terminfo database using the provide terminfo file

tic tmux-256color-it.terminfo

Then uncomment the following line in .tmux.conf

# Enable colors and italics
#set -g default-terminal "tmux-256color-it"