It seems colors only are available for VT100 terminals. I'm using a tmux-256color terminal when I'm running tmux and outside of tmux, the terminal is xterm-256color.
I'm trying to think of where I've seen the use of 256 colors or at least 16 colors in C/C++ programming before, because I know it applies to more than just VT100 terminals.
Support needs to be extended for other terminals.