BarnOwl has built-in support for 256color terminals, assuming your libncursesw (and GNU screen, if you're using it) support it.

If you're running BarnOwl out of the locker and want 256color support, use barnowl-256color instead of barnowl.

As of Debian Lenny, the stock screen is built with 256color support, but it is disabled by default. To enable it, add the following to your ~/.screenrc.

# tell screen how to set colors. AB = background, AF=foreground
termcapinfo xterm 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm'
# erase background with current bg color
defbce "on"

For older platforms, the barnowl locker may have a newer screen under the name of screen-256color; /mit/barnowl/etc/screenrc includes the aforementioned additions.