As of version 2.2 of Tmux, using RGB colors is possible setting
TrueColor for the desired termcap. Also, tmux-256color termcap has been
added to ncurses-term package.
As result of default terminal settings changing as of version 2.1 of
Tmux some terminals, specially over SSH connections, won't detect the
TERM correctly, resulting in wrong colors displayed when using them
inside Tmux (for instance when running Vim).
The Tmux setting using a custom termcap and the shell settings described
in comments help to overcome those issues.