The developers of bash never showed a lot of interest for the feature you request (http://lists.gnu.org/archive/html/bug-bash/2009-08/msg00019.html).
You might try fizsh instead, which provides a zsh implementation of syntax-highlighting. Fizsh is available in Debian unstable ( http://packages.debian.org/sid/shells/fizsh) The package fizsh uses zsh under the hood. Therefore it generally accepts bash-compatible syntax. Moreover, fizsh gives yoy all the benefits of zsh. The latter is IMO a much more powerful shell than bash. Best wishes, Guido
