[JeremyNT]

You need to set your tmux.conf to use the correct term type. For modern distributions it should be

set -g default-terminal "tmux-256color"

You also need to make sure tmux understands that your terminal supports true color mode. Set the following if e.g. using gnome terminal (which calls itself xterm-256color in $TERM)

set-option -ga terminal-overrides ",256col:Tc"

You'll need to reset $TERM to something more standard if you plan to SSH to older systems, though. I have a shell function that wraps SSH and sets the term type to screen for when I log into old systems.