enscript --color --language=html -Eruby --output=dest.html file.rb
You may need to obtain ruby.st for Ruby syntax highlighting:
$ cd /usr/share/enscript/hl $ sudo curl -O http://java.thn.htu.se/~toor/blog-ng/wp-content/ruby.st
Like any good UNIX program, you can also use pipes:
$ find . -name '*.rb' | xargs cat | enscript --color --language=html -Eruby --output=-