I wanted my bash prompt colors in tmux on Ubuntu 11.10 Oneiric Ocelot. The internet didn’t turn up anything 100% right, but after some trial and error, this worked for me: just add the following line to your .tmux.conf:

set -g default-terminal "xterm-color"

This will make sure the following lines in the default .bashrc are triggered:

case "$TERM" in xterm-color) color_prompt=yes;; esac

Hope this helps someone.