There is an an emacs interface to ruby-debug. See http://bashdb.sourceforge.net/rdebug-emacs.html

The rewrite of ruby-debug for Rubinius currently supports syntax highlighting via CodeRay. See https://github.com/rocky/rbx-trepanning or https://github.com/rocky/rbx-trepanning/wiki/Terminal-Colors for how to set that up.

The github project Emacs interface for the newer series of debuggers is https://github.com/rocky/emacs-dbgr.

But as someone else pointed out, it sounds like you'd be happier using an IDE such as one from JetBrains, Aptana, or Eclipse.